Development of a Novel DNA Mono-alkylator Platform for Antibody-Drug Conjugates

Although microtubule inhibitors (MTI) remain a therapeutically valuable payload option for antibody-drug conjugates (ADC), some cancers do not respond to MTI-based ADCs. Efforts to fill this therapeutic gap have led to a recent expansion of the ADC payload "toolbox" to include payloads with novel mechanisms of action such as topoisomerase inhibition and DNA cross-linking. We present here the development of a novel DNA mono-alkylator ADC platform that exhibits sustained tumor growth suppression at single doses in MTI-resistant tumors and is well tolerated in the rat upon repeat dosing. A phosphoramidate prodrug of the payload enables low ADC aggregation even at drug-to-antibody ratios of 5:1 while still delivering a bystander-capable payload that is effective in multidrug resistant (MDR)-overexpressing cell lines. The platform was comparable in xenograft studies to the clinical benchmark DNA mono-alkylator ADC platform DGN459 but with a significantly better tolerability profile in rats. Thus, the activity and tolerability profile of this new platform make it a viable option for the development of ADCs.

Site-Specific Dolasynthen Antibody-Drug Conjugates Exhibit Consistent Pharmacokinetic Profiles across a Wide Range of Drug-to-Antibody Ratios

Key defining attributes of an antibody-drug conjugate (ADC) include the choice of the targeting antibody, linker, payload, and the drug-to-antibody ratio (DAR). Historically, most ADC platforms have used the same DAR for all targets, regardless of target characteristics. However, recent studies and modeling suggest that the optimal DAR can depend on target expression level and intratumoral heterogeneity, target internalization and trafficking, and characteristics of the linker and payload. An ADC platform that enables DAR optimization could improve the success rate of clinical candidates. Here we report a systematic exploration of DAR across a wide range, by combining THIOMAB protein engineering technology with Dolasynthen, an auristatin-based platform with monomeric and trimeric variants. This approach enabled the generation of homogeneous, site-specific ADCs spanning a discrete range of DARs 2, 4, 6, 12, and 18 by conjugation of trastuzumab IgG1 THIOMAB constructs with 1, 2, or 3 engineered cysteines to monomeric or trimeric Dolasynthen. All ADCs had physicochemical properties that translated to excellent in vivo pharmacology. Following a single dose of ADCs in a HER2 xenograft model with moderate antigen expression, our data demonstrated comparable pharmacokinetics for the conjugates across all DARs and dose-dependent efficacy of all test articles. These results demonstrate that the Dolasynthen platform enables the generation of ADCs with a broad range of DAR values and with comparable physiochemical, pharmacologic, and pharmacokinetics profiles; thus, the Dolasynthen platform enables the empirical determination of the optimal DAR for a clinical candidate for a given target.

Discovery and Preclinical Characterization of XMT-1660, an Optimized B7-H4-Targeted Antibody-Drug Conjugate for the Treatment of Cancer

Antibody-drug conjugates (ADC) achieve targeted drug delivery to a tumor and have demonstrated clinical success in many tumor types. The activity and safety profile of an ADC depends on its construction: antibody, payload, linker, and conjugation method, as well as the number of payload drugs per antibody [drug-to-antibody ratio (DAR)]. To allow for ADC optimization for a given target antigen, we developed Dolasynthen (DS), a novel ADC platform based on the payload auristatin hydroxypropylamide, that enables precise DAR-ranging and site-specific conjugation. We used the new platform to optimize an ADC that targets B7-H4 (VTCN1), an immune-suppressive protein that is overexpressed in breast, ovarian, and endometrial cancers. XMT-1660 is a site-specific DS DAR 6 ADC that induced complete tumor regressions in xenograft models of breast and ovarian cancer as well as in a syngeneic breast cancer model that is refractory to PD-1 immune checkpoint inhibition. In a panel of 28 breast cancer PDXs, XMT-1660 demonstrated activity that correlated with B7-H4 expression. XMT-1660 has recently entered clinical development in a phase I study (NCT05377996) in patients with cancer.

Discovery and Optimization of a STING Agonist Platform for Application in Antibody Drug Conjugates

While STING agonists have proven to be effective preclinically as anti-tumor agents, these promising results have yet to be translated in the clinic. A STING agonist antibody-drug conjugate (ADC) could overcome current limitations by improving tumor accessibility, allowing for systemic administration as well as tumor-localized activation of STING for greater anti-tumor activity and better tolerability. In line with this effort, a STING agonist ADC platform was identified through systematic optimization of the payload, linker, and scaffold based on multiple factors including potency and specificity in both in vitro and in vivo evaluations. The platform employs a potent non-cyclic dinucleotide STING agonist, a cleavable ester-based linker, and a hydrophilic PEG8-bisglucamine scaffold. A tumor-targeted ADC built with the resulting STING agonist platform induced robust and durable anti-tumor activity and demonstrated high stability and favorable pharmacokinetics in nonclinical species.

Abstract 3503: XMT-2056, a HER2-targeted Immunosynthen STING-agonist antibody-drug conjugate, binds a novel epitope of HER2 and shows increased anti-tumor activity in combination with trastuzumab and pertuzumab

We present here a novel therapeutic agent, XMT-2056, that results in robust anti-tumor activity mediated by an immune response through targeted delivery of a STING agonist to the tumor microenvironment. By leveraging an antibody-drug conjugate (ADC) strategy, systemic administration of a STING agonist with tumor-targeted delivery can be achieved, potentially overcoming limitations of either intratumoral or intravenous administrations of unconjugated, small molecule STING agonists. XMT-2056 was generated through conjugation of Immunosynthen, a platform that employs a novel STING agonist payload specifically designed for ADCs, to HT-19, a HER2-targeting antibody which binds to a novel epitope and does not compete for binding with either trastuzumab or pertuzumab. Initial results showed XMT-2056 has target-dependent anti-tumor activity in vivo and is well tolerated in non-human primates at significantly higher exposure levels than those required for anti-tumor activity. To evaluate the impact of HER2 expression level on the activity of XMT-2056, in vivo studies in gastric and breast cancer models with varying HER2 expression levels were conducted, and XMT-2056 showed potent anti-tumor activity in a dose dependent and target dependent manner including in models with very low expression of HER2. Because the antibody employed in XMT-2056 does not compete for binding with trastuzumab or pertuzumab, we hypothesized that there could be benefit in combining with such approved HER2-targeted therapies. This advantage was demonstrated in vivo as the combination of XMT-2056 and trastuzumab or pertuzumab showed greater anti-tumor activity compared to the administration of either agent alone. Further efforts to elucidate the mechanism(s) of the observed benefit of these combinations will be discussed. Given the innate immune activation by XMT-2056, there is also a strong rationale for combination with immune checkpoint inhibitors. To this end, administration of an XMT-2056 surrogate ADC in combination with an anti-PD1 agent improved anti-tumor activity in a ratHER2-engineered EMT-6 syngeneic mouse model. Together these data support the potential of XMT-2056 both as a monotherapy and in combination with other HER2 targeted agents as well as checkpoint inhibitors.

Citation Format: Jeremy R. Duvall, Raghida A. Bukhalid, Naniye M. Cetinbas, Kalli C. Catcott, Kelly Lancaster, Keith W. Bentley, Suzanna Clark, Susan Clardy, Scott D. Collins, Anouk Dirksen, Elizabeth Ditty, Bingfan Du, Eugene W. Kelleher, Travis Monnell, Marina Protopopova, Caitlin Routhier, Cheri Stevenson, Elena Ter-Ovanesyan, Joshua D. Thomas, Alex Uttard, Jason Wang, Phonphimon Wongthida, Ling Xu, Annika Yau, Jeffrey Zurita, Dorin Toader, Marc Damelin, Timothy B. Lowinger. XMT-2056, a HER2-targeted Immunosynthen STING-agonist antibody-drug conjugate, binds a novel epitope of HER2 and shows increased anti-tumor activity in combination with trastuzumab and pertuzumab [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3503.

Abstract 2114: Tumor cell-targeted STING-agonist antibody-drug conjugates achieve potent anti-tumor activity by delivering STING agonist specifically to tumor cells andFcγRI-expressing subset of myeloid cells

STING pathway agonism has emerged as a potential therapeutic strategy to stimulate anti-tumor immune responses. We have previously shown that tumor cell-targeted antibody-drug conjugates (ADCs) carrying a novel STING agonist induce anti-tumor activity without causing substantial elevations in systemic cytokine levels, thus suggesting a therapeutic advantage of STING agonist ADCs relative to unconjugated agonists. ADCs constitute a proven therapeutic modality that is ideally suited to enable systemic administration and delivery of the conjugated drug to desired cell types within the tumor microenvironment. In addition to delivering STING agonist into the antigen-expressing tumor cells, antigen-bound ADCs deliver STING agonist to tumor-resident myeloid cells through Fcγ receptor (FcγR)-mediated internalization. In this study we investigated the mechanism of FcγR-mediated internalization of the tumor cell-targeted STING-agonist ADCs into myeloid cells and the nature of the subsequent STING pathway activation. We developed flow cytometry-based assays to determine the changes in FcγRI, FcγRII, and FcγRIII cell surface detection levels in the presence of ADCs specifically designed to be either proficient or deficient in FcγR-binding. Combined with functional assays based on co-culture of cancer cells and FcγRI knock out myeloid cells, we identified FcγRI as the major Fcγ receptor that mediates target-bound ADC internalization into myeloid cells in vitro. Even though FcγRI is expressed only by a subset of CD11b+ myeloid cells, tumor cell-targeted ADCs induce greater production of interferons and other cytokines and more potent cancer cell killing than CD11b-targeted-ADCs, which deliver STING agonist into FcγRI- (non-productive) as well as FcγRI+ (productive) myeloid cells. Finally, we demonstrate that myeloid cells within dissociated primary human tumors from multiple donors express FcγRI and are capable of tumor cell killing in response to tumor cell-targeted STING agonist ADCs in vitro. In summary, our data indicate that the ADC-mediated delivery of a STING agonist specifically into FcγRI-expressing myeloid cells efficiently activates innate immune responses in the most relevant immune cell types within the tumor microenvironment.

Citation Format: Naniye Malli Cetinbas, Kalli C. Catcott, Travis Monnell, Jahna Soomer-James, Keith Bentley, Susan Clardy, Bingfan Du, Eoin Kelleher, Marina Protopopova, Cheri Stevenson, Joshua D. Thomas, Alex Uttard, Dorin Toader, Jeremy Duvall, Raghida Bukhalid, Marc Damelin, Timothy B. Lowinger. Tumor cell-targeted STING-agonist antibody-drug conjugates achieve potent anti-tumor activity by delivering STING agonist specifically to tumor cells andFcγRI-expressing subset of myeloid cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2114.

Abstract P167: Site-specific Dolasynthen ADCs demonstrate consistent exposure across a wide range of drug-to-antibody ratios

Key defining attributes of an antibody-drug conjugate (ADC) include the choice of targeting antibody, linker, and the drug-to-antibody ratio (DAR). The choice of DAR, within the constraints of acceptable physicochemical properties for the given platform, is a function of balancing delivery of sufficient payload to targeted cells with the ability to achieve sustained in vivo exposures. Previous reports have described lower DAR mc-VC-MMAE conjugates, DAR = 1-2, that demonstrated higher in vivo exposure and lower clearance when compared to higher DAR (e.g. 4-8) counterparts. In theory, high DAR conjugates may be especially desirable when targeting low antigen expressing tumors or when lower potency payloads are used, as each binding and internalization event results in greater payload delivery. Here we report a systematic exploration of DAR across a much wider range than has been previously reported, by combining THIOMAB® protein engineering technology with the Dolasynthen platform. Homogeneous, site-specific ADCs spanning a discrete range of DARs – 2, 4, 6, 12, and 18 – were made by conjugation of Trastuzumab IgG1 THIOMAB constructs with 1, 2, or 3 engineered cysteines to monomeric or trimeric Dolasynthen. The cytotoxicity of the resulting well-defined ADCs was assessed in vitro in cell lines with high or low expression of HER2 antigen. Pharmacokinetic data for all test articles in mice were generated in tumor bearing mice. In high HER2 expressing cell lines, in vitro cytotoxicity by payload was comparable across DARs. In a lower HER2 expressing system, the higher DAR ADCs performed better. In vivo, our data demonstrated comparable pharmacokinetics for the Dolasynthen conjugates across all DARs. These results illustrate the utility of a DAR ranging platform, such as Dolasynthen when evaluating ADCs as it enables the interrogation of a range of antibody and payload dosing regimens.

Citation Format: Kalli C. Catcott, Susan Clardy, Jack Sadowsky, Rebecca K. Rowntree, Naniye Malli Centibas, Ling Xu, Andy Polson, Kenneth Avocetien, Tyler Carter, Mark Nazzaro, Dokyong "DK" Kim, Thomas H. Pillow, Neelie Zacharias, Cong Wu, Jeffrey Zurita, Elizabeth Ditty, Stephen Bradley, Alex Uttard, Bingfan Du, William S. Sawyer, Doug Leipold, Gail Lewis Phillips, LiuLiang Qin, Kelly Slocum, Geoffrey Del Rosario, Ginny Li, Shang-Fan Yu, David Lee, Radha Iyengar, Marc Damelin, Dorin Toader, Timothy B. Lowinger. Site-specific Dolasynthen ADCs demonstrate consistent exposure across a wide range of drug-to-antibody ratios [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P167.

785 STING-agonist ADCs targeting tumor-associated antigens coordinate immune-mediated killing of antigen-negative cancer cells

Background

The tumor microenvironment is a complex, multicellular system, composed not only of malignant cancer cells but also of a diversity of stromal cells including vascular cells, immune cells, and fibroblasts that support tumorigenesis. Antigens expressed on these cells tend to be widely expressed across a range of malignancies, presenting unique opportunities for development of anti-cancer therapies.

Methods

We have previously demonstrated that STING-agonist antibody-drug conjugates (Immunosynthen ADCs) targeting tumor cell antigens induce target-dependent anti-tumor immune responses in vitro and in vivo. To that effect, we hypothesized that Immunosynthen ADCs targeting tumor-associated antigens would coordinate immune-mediated killing of cancer cells not expressing the tumor-associated antigens (antigen-negative cancer cells) and induce anti-tumor activity.

Results

Herein, we demonstrate that targeting tumor-associated antigens with STING-agonist ADCs activate the STING pathway in immune cells via Fcγ receptor-mediated uptake. In addition, due to the intrinsic ability of certain tumor-associated cells to activate the STING pathway, STING-agonist ADCs targeting those cells can induce STING signaling in both the targeted cells and the immune cells, which constitutes a therapeutic advantage of ADCs that activate the STING pathway. In triple co-cultures of antigen-positive tumor-associated cells, antigen-negative cancer cells, and immune cells, the STING-agonist ADC specifically induced potent cell killing of the antigen-negative cancer cells with minimal impact on the immune and tumor-associated cells, thus representing a non-traditional, yet highly effective mechanism of ADC targeting. In vivo efficacy studies showed that STING-agonist ADCs developed for two tumor-associated antigens induced complete, sustained tumor regressions in syngeneic tumor models and exhibited immunological memory after rechallenge. CD8+ T cells contributed to the anti-tumor activity of the STING-agonist ADCs.

Conclusions

In summary, Immunosynthen STING-agonist ADCs targeting tumor-associated antigens represent a novel approach for ADC-mediated cancer immunotherapy and enable the multifaceted activation of the STING pathway in a tumor-targeted manner beyond tumor antigens.

Abstract 1738: XMT-2056, a well-tolerated, Immunosynthen-based STING-agonist antibody-drug conjugate which induces anti-tumor immune activity

STING pathway agonism has emerged as a potential therapeutic mechanism to stimulate an innate anti-tumor immune response. However, the systemic administration of a free STING agonist may be limited by toxicity, and broad biodistribution may not be ideal. Antibody-drug conjugates (ADCs) constitute a proven therapeutic modality that enables tumor-targeted delivery and thus is ideally suited to systemic administration with reduced toxicity. To develop an optimized STING-agonist ADC platform, we designed a novel STING-agonist specifically tailored for use in an ADC. Determination of the co-crystal structure confirmed that the agonist binds to the closed, or ‘active', conformation of the STING homodimer. The resulting Immunosynthen platform, which was developed specifically for the selected STING agonist payload, was used to generate XMT-2056, a tumor antigen-targeted STING-agonist ADC with excellent drug-like properties and >100-fold increased potency as compared to the free STING-agonist payload. In mice, XMT-2056 induced robust anti-tumor immune activity, with only minimal increases in systemic cytokine levels, and exhibited significant benefit over the benchmark free STING-agonist payload in both regards. Additionally, in vitro and in vivo studies demonstrate that XMT-2056 is able to activate the STING pathway in both tumor-resident immune cells and tumor cells, offering a potential advantage over other innate immune activating pathways. XMT-2056 was well-tolerated in non-human primates at significantly higher exposure levels than those required for anti-tumor activity, and the ADC exhibited favorable pharmacokinetics after repeat doses. Together these data support the clinical development of XMT-2056.

Citation Format: Jeremy R. Duvall, Raghida A. Bukhalid, Naniye M. Cetinbas, Kalli C. Catcott, Kelly Slocum, Kenneth Avocetien, Keith W. Bentley, Stephen Bradley, Susan Clardy, Scott D. Collins, Elizabeth Ditty, Timothy Eitas, Brian D. Jones, Eugene W. Kelleher, Winnie Lee, Travis Monnell, Rebecca Mosher, Marina Protopopova, LiuLiang Qin, Pamela Shaw, Elena Ter-Ovanesyan, Joshua D. Thomas, Phonphimon Wongthida, Ling Xu, Liping Yang, Jeffrey Zurita, Dorin Toader, Marc Damelin, Timothy B. Lowinger. XMT-2056, a well-tolerated, Immunosynthen-based STING-agonist antibody-drug conjugate which induces anti-tumor immune activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1738.

Abstract 907: XMT-1660, a B7-H4-targeted Dolasynthen antibody-drug conjugate for the treatment of breast cancer

XMT-1660 is a novel Dolasynthen-based antibody drug conjugate carrying a DolaLock payload with controlled bystander effect and targeting B7-H4, a tumor antigen that is broadly expressed on the cell surface in breast, ovarian and endometrial cancers. B7-H4 (VTCN1) exerts immunosuppressive effects by suppression of T cell proliferation and is expressed on tumor-associated macrophages (TAMs) as well as epithelial tumor cells. XMT-1660 is comprised of an anti-B7-H4 antibody site-specifically conjugated to Dolasynthen, with a total of 6 DolaLock Auristatin F-HPA (AF-HPA) anti-tubulin payloads per antibody (DAR-6).

To select the optimal ADC, three ADCs using the same antibody and DolaLock payload were compared: site-specific Dolasynthen-based DAR-2 and DAR-6 ADCs, and a stochastically conjugated Dolaflexin-based DAR-12 ADC. In vitro, no significant differences were observed among the 3 ADCs: all exhibited specific recognition of B7-H4 and elicited potent cytotoxicity against B7-H4-expressing cancer cells. In vivo, XMT-1660 consistently exhibited more anti-tumor activity than the other ADCs in TNBC models and ER+/HER2- models after single, equivalent doses based on payload. XMT-1660 demonstrated dose-dependent anti-tumor activity and induced sustained tumor regressions after a single administration. XMT-1660 and the Dolasynthen DAR-2 ADC both exhibited improved pharmacokinetics in mouse relative to the Dolaflexin DAR 12 ADC.

These data indicate that XMT-1660 exhibited a superior preclinical profile to the other ADCs and more generally demonstrate the importance of DAR-ranging studies to identify the optimal antibody-drug conjugate for a given target. These results, as well as results from exploratory toxicology studies in non-human primates, strongly support the clinical development of XMT-1660.

Citation Format: Shawn P. Fessler, Jason Wang, Scott D. Collins, LiuLiang Qin, Kenneth Avocetien, Ling Xu, Ronald Eydelloth, Steven Vonderfecht, Chen-Ni Chin, Steven Bradley, Susan Clardy, Anouk Dirksen, Elizabeth Ditty, Bingfan Du, Dokyong Kim, Rebecca Mosher, Elena Ter-Ovanesyen, Kelly Slocum, Alex Uttard, Phonphimon Wongthida, Jeffrey Zurita, Dorin Toader, Marc Damelin, Timothy B. Lowinger. XMT-1660, a B7-H4-targeted Dolasynthen antibody-drug conjugate for the treatment of breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 907.

Abstract 1773: Tumor cell-intrinsic STING pathway activation leads to robust induction of Type III Interferons and contributes to the anti-tumor activity elicited by STING agonism

STING pathway plays a critical role in inducing anti-tumor immunity by upregulating Type 1 Interferon (IFN) and IFN-stimulated genes within the tumor microenvironment in response to cytosolic nucleic acid ligands. Therefore, the STING pathway agonism has emerged as a potential therapeutic mechanism to stimulate an anti-tumor innate immune response. Intratumorally injected free STING-agonists that are currently being evaluated in the clinic by others have shown limited effects in non-injected lesions. Antibody-drug conjugates (ADCs) constitute a proven therapeutic modality that enables tumor-targeted drug delivery with systemic administration. We have previously demonstrated that the tumor cell-intrinsic STING pathway is activated in the presence of cues from immune cells and contributes to the anti-tumor activity of tumor cell-targeted Immunosynthen STING-agonist ADCs, in which a STING-agonist payload is conjugated to a tumor cell-targeting antibody. Here we investigated the nature of the STING pathway activation in tumor cells and its contribution to the anti-tumor activity elicited by STING agonism. Leveraging ADCs with a wild type (wt) or mutant Fc (deficient in Fcγ receptor -FcγR- binding), we delivered a STING-agonist simultaneously to tumor-resident immune and cancer cells or only to cancer cells through FcγR-mediated and/or tumor antigen-mediated ADC internalization. We utilized these ADCs in in vivo human tumor xenograft models and STING wt or knock out (ko) cancer cell:immune cell co-cultures and evaluated gene expression, cytokine production, and anti-tumor activities induced by STING-agonist ADCs. Surprisingly, Nanostring analysis of the human tumor xenografts from mice treated with tumor cell-targeted STING-agonist ADCs revealed human tumor cell-specific activation of Type III IFNs. In human cancer cell:immune cell co-cultures, treatment with tumor cell-targeted STING-agonist ADCs also led to marked upregulation of Type III IFNs, which was significantly reduced in STING ko cancer cell:immune cell co-cultures, suggesting that the cancer cells may contribute majority of the Type III IFNs downstream of STING pathway activation. Blocking Type III IFNs with neutralizing antibodies in cancer cell:immune cell co-cultures inhibited the production of key cytokines, including Type I IFN, and nearly abolished tumor cell-killing in response to STING-agonist ADC treatment, indicating that the Type III IFNs play an important role in the anti-tumor activity induced by STING activation. These studies reveal a previously underappreciated mechanism of STING agonist anti-tumor activity. The ability of tumor cell-targeted STING-agonist ADCs to activate STING in both tumor cells and in tumor-resident immune cells may represent a significant therapeutic advantage of an Immunosynthen ADC approach to STING agonism.

Citation Format: Naniye Malli Cetinbas, Travis Monnell, Kalli Catcott, Winnie Lee, Pamela Shaw, Kelly Slocum, Kenneth Avocetien, Keith Bentley, Susan Clardy, Brian Jones, Eoin Kelleher, Rebecca Mosher, Joshua D. Thomas, Dorin Toader, Jeremy Duvall, Raghida A. Bukhalid, Marc Damelin, Timothy B. Lowinger. Tumor cell-intrinsic STING pathway activation leads to robust induction of Type III Interferons and contributes to the anti-tumor activity elicited by STING agonism [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1773.

Abstract 6706: Systemic administration of STING agonist antibody-drug conjugates elicit potent anti-tumor immune responses with minimal induction of circulating cytokines

STING pathway agonism has emerged as a potential therapeutic mechanism to stimulate an innate anti-tumor immune response. While in principle systemic administration of a STING agonist would have many therapeutic benefits, including the delivery of STING to all tumor lesions, such an approach may be limited by toxicity. Antibody-drug conjugates (ADCs) constitute a proven therapeutic modality that is ideally suited to enable systemic administration without associated toxicity concerns via a targeted delivery strategy. Herein, we demonstrate that systemically administered STING agonist ADCs have greater anti-tumor activity as well as greatly improved tolerability compared to an intravenously (IV) administered, unconjugated (free) agonist. We generated novel STING agonist ADCs by leveraging our Immunosynthen platform, in which the chemical scaffold for bioconjugation is optimized for the STING agonist, resulting in an ADC that has desirable physicochemical and drug-like properties. We have studied the in vitro activity and mechanism of action of STING agonist ADCs in monoculture and co-culture systems. STING agonist ADCs were at least 100-fold more potent in inducing interferon and cytokines as well as tumor cell-killing relative to free agonist. STING agonist ADCs against several targets (antigens) have been evaluated for anti-tumor activity and pharmacodynamic and pharmacokinetic properties in multiple xenograft and syngeneic models. A single administration of STING agonist ADC resulted in target-dependent, durable, and complete regressions. Importantly, the STING agonist ADC led to an increase in tumor-localized inflammatory cytokines and significant immune cell infiltration, while levels of systemic cytokines remained low. In contrast, IV administered free agonist induced up to 100-fold higher levels of systemic cytokines with concomitant body weight loss but only modest tumor growth delay. In summary, Immunosynthen represents a novel STING agonist ADC platform. We have demonstrated target-dependent anti-tumor immune responses in vitro and in vivo for multiple targets, tumor models, and mouse strains. In each case the STING agonist ADC was more active and better tolerated than the IV administered free agonist.

Citation Format: Raghida A. Bukhalid, Jeremy R. Duvall, Naniye Malli Cetinbas, Kalli C. Catcott, Kenneth Avocetien, Keith W. Bentley, Stephen Bradley, Tyler Carter, Chen-Ni Chin, Susan Clardy, Scott D. Collins, Timothy Eitas, Brian D. Jones, Eugene W. Kelleher, Rebecca Mosher, Mark Nazzaro, Marina Protopopova, Pamela Shaw, Kelly Slocum, Elena Ter-Ovanesyan, LiuLiang Qin, Joshua D. Thomas, Ling Xu, Liping Yang, Jeffrey Zurita, Dorin Toader, Marc Damelin, Timothy B. Lowinger. Systemic administration of STING agonist antibody-drug conjugates elicit potent anti-tumor immune responses with minimal induction of circulating cytokines [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6706.

Abstract 2894: XMT-1592, a site-specific Dolasynthen-based NaPi2b-targeted antibody-drug conjugate for the treatment of ovarian cancer and lung adenocarcinoma

The Dolasynthen platform incorporates the highly potent anti-mitotic agent auristatin F-HPA (AF-HPA), with its associated DolaLock mechanism of controlled bystander effect, and enables the synthesis of antibody-drug conjugates (ADCs) with precise control of the drug-to-antibody ratio (DAR) and site-specific bioconjugation. XMT-1592 is a novel ADC comprised of an anti-NaPi2b antibody and Dolasynthen, conjugated in a site-specific manner to yield DAR 6.

NaPi2b, also known as SLC34A2, is a transmembrane sodium-phosphate transporter that is broadly expressed on tumor cells in ovarian carcinoma, NSCLC lung adenocarcinoma and other tumor types. Recent studies have shown that NaPi2b expression is enriched in the EGFR and KRAS mutant subtypes of lung adenocarcinoma.

Binding studies showed a specific, high-affinity interaction of XMT-1592 with NaPi2b that was not affected by conjugated Dolasynthen. XMT-1592 elicited potent and specific in vitro cytotoxicity against NaPi2b-expressing ovarian carcinoma cells. XMT-1592 exhibited potent and specific in vivo activity in NaPi2b-expressing tumor xenografts derived from ovarian carcinoma or lung adenocarcinoma. Consistent with the targeted delivery benefits of the ADC approach, XMT-1592 yielded high and sustained concentrations of AF-HPA to tumors but not normal tissues.

To evaluate the benefits of site-specific bioconjugation of Dolasynthen, we conducted in vitro and in vivo comparisons of XMT-1592 to a stochastically conjugated version of the ADC. XMT-1592 had improved in vivo activity, pharmacokinetics, and clinical pathology relative to its stochastic counterpart. Taken together, these results support XMT-1592 as a development candidate for the treatment of NaPi2b-expressing tumors.

Citation Format: Shawn Fessler, Anouk Dirksen, Scott D. Collins, Ling Xu, Winnie Lee, Jason Wang, Ron Eydelloth, Elena Ter-Ovanesyen, Jeffrey Zurita, Elizabeth Ditty, Barrett Nehilla, Susan Clardy, Susan Clardy, Tyler Carter, Kenneth Avocetien, Mark Nazzaro, Nam Le, Kalli C. Catcott, Alex Uttard, Bingfan Du, Chen-Ni Chin, Rebecca Mosher, Kelly Slocum, Liuliang Qin, David Lee, Dorin Toader, Marc Damelin, Timothy B. Lowinger. XMT-1592, a site-specific Dolasynthen-based NaPi2b-targeted antibody-drug conjugate for the treatment of ovarian cancer and lung adenocarcinoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2894.

Abstract 2687: Dolasynthen–a novel, homogeneous Auristatin F hydroxypropyl amide antibody-drug conjugate platform

Dolasynthen is a novel, fully synthetic, structurally homogeneous platform that enables the construction of ADCs with tunable drug-to-antibody ratios (DAR), from a low of 2 to a high of 24. The resulting ADCs exhibit excellent physicochemical properties and fully homogeneous conjugates can be created through a variety of bioconjugation chemistries. Analogous to our first platform, Dolaflexin®, Dolasynthen is loaded with the proprietary payload Auristatin F hydroxypropylamide (AF-HPA) with precisely defined numbers of the cytotoxin per Dolasynthen scaffold.Studies that evaluate the tolerability and efficacy of Dolasynthen in preclinical models are described herein. ADCs containing a range of DAR values were generated following conjugation of Dolasynthen to two different monoclonal antibodies. The DAR of the ADCs was achieved by controlled reduction of native disulfide bonds in IgG1 antibodies, chromatographic fractionation, or through use of site-specific conjugation technologies. ADCs with both DAR6 and DAR12 were evaluated in vitro and also in vivo in the mouse, rat and monkey, for efficacy, tolerability and PK. Dolasynthen conjugates had excellent physicochemical properties and displayed the expected cell binding and in vitro cytotoxicities. In vivo pharmacology of Dolasynthen ADCs in in vivo xenograft models showed dose-dependent tumor growth inhibition at low mg/kg mAb doses. Tolerability in the rat at multiple doses was determined, including histopathological evaluation. Dolasynthen ADCs showed excellent PK characteristics in mouse, rat and NHP. Overall, the Dolasynthen platform appears to offer significant potential for clinical application.

Citation Format: Dorin Toader, Marc Damelin, Anouk Dirksen, Shawn P. Fesler, Scott D. Collins, Barrett J. Nehilla, Jian Xu, Ling Xu, Kalli C. Cattcott, Alex Uttard, Winnie Lee, Susan Clardy, Cheri A. Stevenson, LiuLiang Qin, Patrick R. Conlon, Mariya V. Kozytska, Chen-Ni Chin, David H. Lee, Timothy B. Lowinger. Dolasynthen–a novel, homogeneous Auristatin F hydroxypropyl amide antibody-drug conjugate platform [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2687.

Abstract 232: An antibody-drug conjugate carrying a microtubule inhibitor and a DNA alkylator exerts both mechanisms of action on tumor cells

Antibody-drug conjugates (ADCs) consist of a tumor-targeted antibody, a drug (payload) with specified mechanism of action, and the chemical framework for attaching them to each other. By selective delivery of the payload to the tumor and not to normal tissues, ADCs can provide greater efficacy and tolerability than systemic chemotherapies, which can translate to longer duration of treatment and response, as well as more options for combination therapies without the concern of overlapping toxicity. To preempt potential resistance to therapy, we have engineered a dual-payload ADC (DP-ADC) that delivers two mechanistically distinct payloads to a single target cell.

To build a precision dual payload-ADC (DP-ADC), we leveraged our Synthemer platform, which enables us to chemically attach the payloads to a synthetic scaffold in a defined manner, and then to chemically attach the loaded scaffold to the antibody. This approach allows for unambiguous control of architecture and ratio of the payloads: the payloads are incorporated in a fixed ratio into the scaffold and subsequently into the ADC. Moreover, the synthetic design also allows for selection of solubilizing groups and charge compensation for each payload, allowing for optimal properties of the ADC.

Our prototype DP-ADC combines the microtubule inhibitor auristatin F hydroxypropyl amide (AF-HPA) with the DNA monoalkylator I-BiP (related to pyrrolobenzodiazepine), at a payload ratio of 3:1. The drug-to-antibody ratio (DAR) was 12 AF-HPA and 4 I-BiP (16 total), achieved by conjugating 4 of the loaded dual-payload scaffolds to the antibody. The DP-ADC exhibited comparable antigen binding to the unconjugated antibody, and cytotoxic potency comparable to both single-payload ADCs alone. Using cell-based assays that specifically probe the mechanism of action of each payload, we demonstrated that the DP-ADC exerted both expected mechanisms of action on the tumor cell in a target-dependent manner.

Considerations for the selection of payload combinations will also be presented. While synergistic action is typically assumed to be preferred if not required, a recent study demonstrated that the observed clinical benefit of most combinations can be explained by the drugs’ independent modes of action, not synergy. Indeed, AF-HPA and I-BiP did not exhibit synergy in cytotoxicity studies in cancer cell lines, yet their combination in a DP-ADC is still expected to confer potential clinical benefit over either single-payload ADC across a patient population.

Citation Format: Jeremy R. Duvall, Marc Damelin, Mariya V. Kozytska, Barrett J. Nehilla, Marina Protopopova, Patrick R. Conlon, LiuLiang Qin, Mark Nazzaro, Josh D. Thomas, Qingxiu Zhang, Dorin Toader, Timothy B. Lowinger. An antibody-drug conjugate carrying a microtubule inhibitor and a DNA alkylator exerts both mechanisms of action on tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 232.

Abstract 754: Unique pharmacologic properties of Dolaflexin-based ADCs—a controlled bystander effect

Antibody-drug conjugates (ADCs) are designed to bind tumor-associated antigens and deliver conjugated cytotoxic payloads to antigen-positive cells. Some ADCs also kill neighboring cells (including antigen-negative cells) by a mechanism referred to as the bystander effect. This effect can be beneficial when the antigen has heterogeneous expression among cells in a solid tumor, but it can also increase off-target toxicity of ADCs. Herein, we report on a unique pharmacologic property of the Dolaflexin platform, which provides a controlled bystander effect that retains the benefits of the bystander effect with respect to antitumor cytotoxicity but reduces the off-target toxicity. The controlled bystander effect, termed “Dolalock,” was achieved through design of a payload, auristatin F-hydroxypropylamide (AF-HPA), that is membrane-permeable and capable of bystander killing but is further catabolized to membrane-impermeable auristatin F (AF). This catabolism of the payload “locks” the highly potent AF in the cell. Using Dolaflexin-based ADCs, we investigated the extent of intracellular AF-HPA and AF release, tumor cell retention and bystander activities in vitro and in vivo. We observed both auristatin species within cells. Co-culture assays with HER2-positive and HER2-negative cells confirmed the cell permeability and bystander-killing capabilities of AF-HPA released from a Dolaflexin-based ADC. Biodistribution studies of Dolaflexin-based ADCs revealed time-dependent concentrations of AF-HPA and AF as well as significant accumulation of AF in xenografted tumor cells, consistent with the Dolalock mechanism. An additional benefit of AF formation was seen in multidrug-resistant transporter studies, which demonstrate that AF, in contrast to AF-HPA, is not a P-glycoprotein 1 (Pgp) substrate. This property may offer additional benefit in Pgp-expressing tumors. In summary, we have shown that the novel AF-HPA payload used in the Dolaflexin platform allows for a controlled bystander effect that likely contributes to the enhanced efficacy and lack of neutropenia we have observed with Dolaflexin-based ADCs.

Citation Format: Susan M. Clardy, Alex Yurkovetskiy, Mao Yin, Dmitry Gumerov, Ling Xu, Elena Ter-Ovanesyan, Charlie Bu, Alex Johnson, Marina Protopopova, Qingxiu Zhang, Natalya Bodyak, Marc Damelin, David H. Lee, Donald Bergstrom, Timothy B. Lowinger. Unique pharmacologic properties of Dolaflexin-based ADCs—a controlled bystander effect [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 754.

Abstract 935: Generation and preclinical characterization of CD123-CPI antibody-drug conjugate (ADC)

CD123 is expressed on cancer cells in a variety of hematologic malignancies including acute myeloid leukemia (AML). CD123 is frequently expressed on leukemic blasts and leukemic stem cells (LSCs), a cell population associated with relapse in patients, but there is minimal or no expression on most normal hematopoietic cells and solid tissues. Thus, CD123 is a promising target for AML. We have generated an anti-CD123 ADC that carries cyclopropylpyrroloindoline (CPI) payload that crosslinks DNA. A drug loading of 2 molecules of CPI per molecule of antibody was achieved by site-specific conjugation using our transglutaminase methodology. Upon binding to the CD123 antigen, CD123-ADC is internalized and delivered to the endosomal-lysosomal pathway whereupon the CPI payload is released from the antibody by proteolysis of the linker. The released CPI alkylates DNA, which activates ATR/ATM, CHK1, CHK2 and FANCD2, ultimately resulting in cell death. In vitro, CD123–CPI elicited cytotoxicity in a dose-dependent manner against several CD123-positive, but not against CD123-negative cell lines. Cell lines with higher CD123 expression level were more sensitive to the ADC. Long-Term Culture-Initiating Cells in vitro assay showed that AML patient bone marrow samples that naturally have high percentage of LSCs yielded substantially fewer colonies in CD123-ADC treated cells compared to control ADC. Importantly, CD123-ADC had no adverse effects in healthy donor bone marrow cells experimentally enriched in CD34+ primitive stem cells. This result suggests that CD123-ADC specifically inhibits the growth potential of leukemic blasts and progenitor cells. In vivo, robust antileukemic activity was observed in CD123-positive AML cell line-derived xenograft models. Low doses of CD123-ADC effectively regressed tumors whereas the tumor progressed in mice that received control ADC. Efficacy was also evaluated in disseminated AML PDX models (n≥7) established with patient samples of various cytogenetics and molecular abnormalities, and relapse/refractory. Flow cytometry analyses of leukemic load in peripheral blood and in bone marrow samples of mice showed that CD123-ADC was efficacious in reducing tumor burden even at the suboptimal doses. In summary, our data demonstrate that our CD123-ADC is highly active in a broad panel of primary AML samples. Currently, CD123-ADC in combination with other AML therapies is being tested in vivo. All these attributes of CD123-ADC make it an attractive agent to evaluate in clinical trials.

Citation Format: Yoon-Chi Han, Fan Jiang, Nicole Piche-Nicholas, Madan Katragadda, Nadira Prashad, Manoj Charati, Wendy Hu, Mauricio Leal, Nathan Tumey, Andreas Maderna, Russell Dushin, Kenny Kim, LuAnna Lemon, Marc Damelin, HP Gerber, Lioudmila Tchistiakova, Frank Loganza, Chris O'Donnell, Puja Sapra. Generation and preclinical characterization of CD123-CPI antibody-drug conjugate (ADC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 935.

Upregulation of RNA Processing Factors in Poorly Differentiated Lung Cancer Cells

Intratumoral heterogeneity in non–small cell lung cancer (NSCLC) has been appreciated at the histological and cellular levels, but the association of less differentiated pathology with poor clinical outcome is not understood at the molecular level. Gene expression profiling of intact human tumors fails to reveal the molecular nature of functionally distinct epithelial cell subpopulations, in particular the tumor cells that fuel tumor growth, metastasis, and disease relapse. We generated primary serum-free cultures of NSCLC and then exposed them to conditions known to promote differentiation: the air-liquid interface (ALI) and serum. The transcriptional network of the primary cultures was associated with stem cells, indicating a poorly differentiated state, and worse overall survival of NSCLC patients. Strikingly, the overexpression of RNA splicing and processing factors was a prominent feature of the poorly differentiated cells and was also observed in clinical datasets. A genome-wide analysis of splice isoform expression revealed many alternative splicing events that were specific to the differentiation state of the cells, including an unexpectedly high frequency of events on chromosome 19. The poorly differentiated cells exhibited alternative splicing in many genes associated with tumor progression, as exemplified by the preferential expression of the short isoform of telomeric repeat-binding factor 1 (TERF1), also known as Pin2. Our findings demonstrate the utility of the ALI method for probing the molecular mechanisms that underlie NSCLC pathogenesis and provide novel insight into posttranscriptional mechanisms in poorly differentiated lung cancer cells.

Abstract 5425: An anti-Ephrin-A4 calicheamicin conjugate effectively targets triple-negative breast and ovarian tumor-initiating cells to result in sustained tumor regression

Triple-negative breast cancer (TNBC) and ovarian cancer comprise heterogeneous tumors, and neither targeted therapies nor traditional chemotherapies have provided consistent clinical benefit. Novel therapies that target and actively eradicate the subpopulation of tumor cells that mediate drug resistance and tumor relapse could significantly improve patient survival. Tumor-initiating cells (TIC) are functionally defined as the subpopulation of cells that drive long-term tumor growth, resistance to therapy and disease relapse. We herein identified CD324 as a surface antigen able to reproducibly enrich for TIC in well annotated, low passage TNBC and ovarian cancer patient-derived xenografts (PDXs). Gene expression analysis of TIC led to the identification of Ephrin-A4 as a prospective therapeutic TIC target. Humanized Ephrin-A4-specific monoclonal antibodies (mAbs) were generated and demonstrated to internalize to mediate the delivery of potent cytotoxins. An antibody-drug conjugate (ADC) comprising a humanized anti-Ephrin-A4 mAb conjugated to the DNA damaging agent calicheamicin achieved sustained tumor regressions in vivo in both TNBC and ovarian cancer PDX. Anti-Ephrin-A4-ADC (PF-06647263) actively reduced TIC frequency as evidenced by limiting dilution analysis in serial transplantation assays. Unexpectedly, TNBC tumors of the non-Claudin low molecular subtype exhibited higher Ephrin-A4 expression and more robust responses to the ADC than other breast cancer subtypes, which suggests a specific translational application for breast tumor subtype classification. Together these findings demonstrate the potential of the Ephrin-A4-targeted calicheamicin conjugate as a first-in-class compound designed to eradicate TIC and improve long-term survival of cancer patients. PF-06647263 is currently being evaluated in a Phase I clinical trial.

Citation Format: Marc Damelin, Alex Bankovich, Albert Park, Jorge Aguilar, Wade Anderson, Marianne Santaguida, Sarah Fong, Monette Aujay, Kiran Khandke, Virginia Pulito, Elana Ernstoff, Paul Escarpe, Jeff Bernstein, Marybeth A. Pysz, Wenyan Zhong, Erik Upeslacis, Judy Lucas, Justin Lucas, Timothy Nichols, Kathryn Loving, Orit Foord, Johannes Hampl, Robert Stull, Frank Barletta, Hadi Falahatpisheh, Puja Sapra, Hans Peter Gerber, Scott J. Dylla. An anti-Ephrin-A4 calicheamicin conjugate effectively targets triple-negative breast and ovarian tumor-initiating cells to result in sustained tumor regression. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5425. doi:10.1158/1538-7445.AM2015-5425

Anti-EFNA4 Calicheamicin Conjugates Effectively Target Triple-Negative Breast and Ovarian Tumor-Initiating Cells to Result in Sustained Tumor Regressions

PURPOSE

Triple-negative breast cancer (TNBC) and ovarian cancer each comprise heterogeneous tumors, for which current therapies have little clinical benefit. Novel therapies that target and eradicate tumor-initiating cells (TIC) are needed to significantly improve survival.

EXPERIMENTAL DESIGN

A panel of well-annotated patient-derived xenografts (PDX) was established, and surface markers that enriched for TIC in specific tumor subtypes were empirically determined. The TICs were queried for overexpressed antigens, one of which was selected to be the target of an antibody-drug conjugate (ADC). The efficacy of the ADC was evaluated in 15 PDX models to generate hypotheses for patient stratification.

RESULTS

We herein identified E-cadherin (CD324) as a surface antigen able to reproducibly enrich for TIC in well-annotated, low-passage TNBC and ovarian cancer PDXs. Gene expression analysis of TIC led to the identification of Ephrin-A4 (EFNA4) as a prospective therapeutic target. An ADC comprising a humanized anti-EFNA4 monoclonal antibody conjugated to the DNA-damaging agent calicheamicin achieved sustained tumor regressions in both TNBC and ovarian cancer PDX in vivo. Non-claudin low TNBC tumors exhibited higher expression and more robust responses than other breast cancer subtypes, suggesting a specific translational application for tumor subclassification.

CONCLUSIONS

These findings demonstrate the potential of PF-06647263 (anti-EFNA4-ADC) as a first-in-class compound designed to eradicate TIC. The use of well-annotated PDX for drug discovery enabled the identification of a novel TIC target, pharmacologic evaluation of the compound, and translational studies to inform clinical development.

Phenotype of TPBG Gene Replacement in the Mouse and Impact on the Pharmacokinetics of an Antibody-Drug Conjugate

The use of predictive preclinical models in drug discovery is critical for compound selection, optimization, preclinical to clinical translation, and strategic decision-making. Trophoblast glycoprotein (TPBG), also known as 5T4, is the therapeutic target of several anticancer agents currently in clinical development, largely due to its high expression in tumors and low expression in normal adult tissues. In this study, mice were engineered to express human TPBG under endogenous regulatory sequences by replacement of the murine Tpbg coding sequence. The gene replacement was considered functional since the hTPBG knockin (hTPBG-KI) mice did not exhibit clinical observations or histopathological phenotypes that are associated with Tpbg gene deletion, except in rare instances. The expression of hTPBG in certain epithelial cell types and in different microregions of the brain and spinal cord was consistent with previously reported phenotypes and expression patterns. In pharmacokinetic studies, the exposure of a clinical-stage anti-TPBG antibody-drug conjugate (ADC), A1mcMMAF, was lower in hTPBG-KI versus wild-type animals, which was evidence of target-related increased clearance in hTPBG-KI mice. Thus, the hTPBG-KI mice constitute an improved system for pharmacology studies with current and future TPBG-targeted therapies and can generate more precise pharmacokinetic and pharmacodynamic data. In general the strategy of employing gene replacement to improve pharmacokinetic assessments should be broadly applicable to the discovery and development of ADCs and other biotherapeutics.

Abstract 1968: Breast cancer cells escape from chemotherapy and hypoxia by distinct mechanisms

Cancer relapse following treatment remains a significant barrier to achieving cures for many patients. An emerging framework for addressing this problem focuses on cancer stem cells (CSCs). Clinical and preclinical data suggest that CSCs survive chemotherapy and antiangiogenic therapy. In breast cancer, CSCs are marked by the cell surface expression of CD44, the major receptor for hyaluronic acid. The SKBR3 cell line was previously shown to be a clinically relevant model of breast CSCs in vivo, and we have further developed this model to enable mechanistic studies of tumor relapse in vitro. We found that chemotherapy and hypoxia both enriched for CD44hi populations in SKBR3, but surprisingly, the populations were phenotypically distinct. CD44hi cells from chemotherapy but not hypoxia exhibited increased tumor cell growth and increased sensitivity to the CSC-specific inhibitor salinomycin, compared to CD44lo cells. To examine these CD44hi and CD44lo populations further, we performed transcriptional profiling with sorted cells. We found that the growth factor TGF beta-2 was upregulated in chemotherapy-treated CD44hi cells and also enhanced the growth of these cells, which suggests that TGF beta-2 autocrine/paracrine signaling can promote the growth of surviving cells. We also observed an increase in xCT (SLC7A11) expression upon chemotherapy treatment and identified the CD44v8-10 variant expressed in the CD44hi SKBR3. CD44v could therefore stabilize xCT and promote survival by lowering intracellular reactive oxygen species (ROS). We have also established a functional role for CD44 in SKBR3 cell growth: CD44 knockdown prevented colony formation, and conversely the CD44 ligand hyaluronic acid enhanced colony formation. These findings indicate that CD44 not only marks CSC populations that arise in response to chemotherapy but also functions in their survival. Our work suggests that mechanisms of tumor relapse vary based on the particular therapy, and defining these mechanisms will allow for the development of novel therapeutic strategies to enable long-term responses in the clinic.

Citation Format: Siobhan O'Brien, Liang Chen, Wenyan Zhong, Douglas Armellino, Maximillian Follettie, Marc Damelin. Breast cancer cells escape from chemotherapy and hypoxia by distinct mechanisms. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1968. doi:10.1158/1538-7445.AM2014-1968

Abstract 2059: In situ imaging of antibody drug conjugate (ADC) binding and pharmacodynamic biomarkers of response in models of human cancer

Antibody drug conjugates (ADCs) are clinically validated as a modality for targeted therapy of solid and hematological cancer due to advancements in target selection, conjugation chemistry and linker technology. However, much about mechanism of action (MoA) is yet to be fully understood. Our goal was to interrogate ADC pharmacokinetics and pharmacodynamics establishing proof of mechanism (PoM) of drug action with a diverse panel of ADCs. Herein, we describe the development of novel immunohistochemical (IHC) methods for in situ visualization of ADCs binding to target expressing cells and their cognate downstream biomarkers of response in formalin fixed paraffin embedded cells/tissues. We demonstrate specific binding of 4 different ADCs spanning 2 solid tumor targets and an endothelial cell target using IHC with anti-human IgG in human tumor xenograft models expressing the respective targets. ADC binding to target is observed as early as 20 minutes after a single dose of ADC at 3 mg/kg. Utilizing an anti-microtubule inhibitor (MTI) payload-specific antibody we additionally detect ADC binding to tumor cells by monitoring the cytotoxic payload. The cell type where the antibodies and payload localized was identified by double and triple IHC. Pharmacodynamic biomarkers of response for two payload classes (DNA damaging agents and MTIs) were detected with antibodies against phospho-Histone H2AX and phospho-Histone H3, respectively - confirming the expected ADC MoAs. Downstream apoptosis of target cells was detected with cleaved caspase 3 IHC. The kinetics of biomarker response and downstream cellular impact was quantified via image analysis with biomarkers evident as early as 24 hours after a single dose for both tumor cell and vascular targets. Furthermore, we observed a correlation between biomarkers of response and efficacy of the ADCs as measured by statistically significant tumor growth inhibition for the 4 ADCs we studied. These data suggest that IHC interrogations of drug action should be used to further the clinical development of ADCs via demonstration of pharmacodynamic activities at the cellular level, establishing PoM data, and enabling predictive preclinical oncology models in order to reduce clinical attrition of ADCs.

Citation Format: Jonathon Golas, Andrea T. Hooper, Justin Lucas, Heather Jones, Timothy Nichols, Kiran Khandke, Manoj Charati, Roger Conant, Michael Cinque, Judy Lucas, Marc Damelin, Ken Geles, Caiazzo Teresa, Frank Loganzo, Puja Sapra, Hans-Peter Gerber, Chad May. In situ imaging of antibody drug conjugate (ADC) binding and pharmacodynamic biomarkers of response in models of human cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2059. doi:10.1158/1538-7445.AM2014-2059

Abstract 4752: Preclinical development and translational research on a novel antibody-drug conjugate that targets 5T4, an oncofetal antigen expressed on tumor-initiating cells

Antibody-drug conjugates (ADCs) represent a promising therapeutic modality for the clinical management of cancer. We sought to develop a novel ADC that targets 5T4 (TPBG), an oncofetal antigen expressed on tumor-initiating cells (TICs), which comprise the most aggressive cell population in the tumor. We optimized an anti-5T4 ADC (A1mcMMAF) by sulfydryl-based conjugation of the humanized A1 antibody to the tubulin inhibitor monomethylauristatin F (MMAF) via a maleimidocaproyl linker. A1mcMMAF exhibited potent in vivo anti-tumor activity in a variety of tumor models and induced long-term regressions for up to 100 days after the last dose. Strikingly, animals showed pathological complete response in each model. In a non-small cell lung cancer patient-derived xenograft in which 5T4 is preferentially expressed on the less differentiated tumor cells, A1mcMMAF treatment resulted in sustained tumor regressions and reduced TIC frequency. These results highlight the potential of ADCs that target the most aggressive cell populations within tumors. An optimized pharmacokinetic/pharmacodynamic (PK/PD) model of tumor growth and drug kill was used to characterize the ADC concentration response relationship in mouse. A holistic secondary parameter, tumor static concentration (TSC), was derived from model parameters to quantify efficacy and support early clinical trial design. Tumor static concentrations [80% confidence] of A1mcMMAF ranged from 1.1[0.9 -1.4] μg/ml to 11.6 [9.6 - 14.1] μg/ml across tumor models. For comparison, in the clinic T-DM1 has an average concentration of 14 μg/ml at an efficacious dose of 3.6 mg/kg Q3wk (HER+ breast cancer) (Krop et al. 2010) and Brentuximab-vedotin has an average concentration of 3.65 μg/ml at an efficacious dose of 1.8 mg/kg Q 3wk (HL/ ALCL) (Younes et al. 2010). Taken together, the preclinical data established a promising therapeutic index that supports clinical testing of A1mcMMAF. Expression analysis profiling using clinical and preclinical data indicated that lung and breast tumors demonstrated differentially high expression of 5T4 in comparison to normal tissues. An IHC assay developed in house confirmed the hypothesis that a broad range of 5T4 expression was measurable in NSCLC patient tumor samples. Additionally, we developed an assay that measures 5T4 expression on circulating tumor cells (CTCs) and used this assay to measure and characterize a broad range of 5T4 expression in CTCs obtained from the blood of NSCLC patients. We intend to deploy these co-developed immunoassays to guide A1mcMMAF clinical development.

Citation Format: Puja Sapra, Marc Damelin, Kimberly Marquette, Kenneth G. Geles, Jonathon Golas, Maureen Dougher, Bitha Narayanan, Andreas Giannakou, Kiran Khandke, Russell Dushin, Elana Ernstoff, Judy Lucas, Mauricio Leal, George Hu, Alison Betts, Nahor Haddish-Berhane, Eric Powell, Steven Pirie-Shepherd, Christopher O'Donnell, Lioudmila Tchistiakova, Hans-Peter Gerber, Dena Marrinucci, Eric Tucker. Preclinical development and translational research on a novel antibody-drug conjugate that targets 5T4, an oncofetal antigen expressed on tumor-initiating cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4752. doi:10.1158/1538-7445.AM2013-4752

Comprehensive interrogation of a minimalist synthetic CDR-H3 library and its ability to generate antibodies with therapeutic potential

We have generated large libraries of single-chain Fv antibody fragments (>10(10) transformants) containing unbiased amino acid diversity that is restricted to the central combining site of the stable, well-expressed DP47 and DPK22 germline V-genes. Library WySH2A was constructed to examine the potential for synthetic complementarity-determining region (CDR)-H3 diversity to act as the lone source of binding specificity. Library WySH2B was constructed to assess the necessity for diversification in both the H3 and L3. Both libraries provided diverse, specific antibodies, yielding a total of 243 unique hits against 7 different targets, but WySH2B produced fewer hits than WySH2A when selected in parallel. WySH2A also consistently produced hits of similar quality to WySH2B, demonstrating that the diversification of the CDR-L3 reduces library fitness. Despite the absence of deliberate bias in the library design, CDR length was strongly associated with the number of hits produced, leading to a functional loop length distribution profile that mimics the biases observed in the natural repertoire. A similar trend was also observed for the CDR-L3. After target selections, several key amino acids were enriched in the CDR-H3 (e.g., small and aromatic residues) while others were reduced (e.g., strongly charged residues) in a manner that was specific to position, preferentially occurred in CDR-H3 stem positions, and tended towards residues associated with loop stabilization. As proof of principle for the WySH2 libraries to produce viable lead candidate antibodies, 114 unique hits were produced against Delta-like ligand 4 (DLL4). Leads exhibited nanomolar binding affinities, highly specific staining of DLL4+ cells, and biochemical neutralization of DLL4-NOTCH1 interaction.

Abstract 2530: Long-term tumor regression induced by a novel antibody drug conjugate that targets 5T4, an oncofetal antigen expressed on tumor-initiating cells

Antibody-drug conjugates (ADCs) represent a promising therapeutic modality for the clinical management of cancer. We have developed a novel ADC that targets 5T4 (also known as TPBG), a tumor-associated antigen that rapidly internalizes. In preclinical models of NSCLC, 5T4 was identified as a marker of undifferentiated tumorigenic cells that express properties of tumor-initiating cells (TICs) and was associated with a highly proliferating (Ki67 positive) cell phenotype. Based on the biological properties of 5T4, we designed the ADC by conjugating an internalizing humanized anti-5T4 antibody (Ab), A1, to the tubulin inhibitor monomethylauristatin F via a maleimidocaproyl (mc) linker (A1mcMMAF) with a drug/antibody ratio of ∼ 4. The A1 Ab has a Kd of 0.9 nM against the 5T4 antigen, binds to a broad range of 5T4 expressing cell lines, and rapidly internalizes (66% internalized within 4h). As an ADC, A1mcMMAF retains similar properties to the unconjugated Ab. In vitro, A1mcMMAF exhibited potent and target-specific cytotoxic activity against a panel of 5T4+ cell lines with an IC50 of 5 ng Ab/ml against high 5T4 expressing cells. In vivo, A1mcMMAF exhibited potent anti-tumor activity in all five tumor models tested (with a broad range of 5T4 expression) and caused long-term regressions monitored up to 100 days after the last dose. Strikingly, animals were cured of disease in every model with doses as low as 3 mg Ab/kg Q4d x 4. In a lung cancer patient-derived xenograft in which 5T4 is not uniformly expressed, but instead is preferentially expressed on the less differentiated sub-population of tumor cells, A1mcMMAF treatment resulted in sustained tumor regression. This result highlights the clinical potential of ADCs that target aggressive cell subpopulations such as TICs. In exploratory safety studies, A1mcMMAF exhibited no overt toxicities when administered at 10 mg Ab/kg/cycle x 2 to cynomolgus monkeys and had a half-life of ∼5 days. The encouraging efficacy and safety data of the conjugate resulting in high therapeutic index in preclinical models warrants further clinical testing of this ADC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2530. doi:1538-7445.AM2012-2530

Abstract A215: Antitumor activity of an antibody-drug conjugate targeting 5T4, a putative tumor-initiating cell antigen

5T4 (also known as TPBG) is a transmembrane, tumor-associated protein that rapidly internalizes. In preclinical models of NSCLC, 5T4 was identified as a marker of undifferentiated tumorigenic cells that express properties of tumor-initiating cells (TICs) and was associated with a highly proliferating (Ki67 positive) cell phenotype. In addition, 5T4 has been associated with a worse clinical outcome in NSCLC (Damelin et al, Cancer Res, 2011). We constructed an ADC with a humanized anti-5T4 Ab (A1) linked to the tubulin inhibitor monomethylauristatin F through the noncleavable maleimidocaproyl (mc) linker attached to cysteine residues (A1mcMMAF) with a DAR of approximately 4 in order to target 5T4 expressing tumor cells. Anti-5T4 A1 Ab has a Kd of 0.9 nM against the 5T4 antigen, binds to a broad range of 5T4 expressing cell lines, and rapidly internalizes (66% internalized within 4h). As an ADC, A1mcMMAF retains similar properties to the unconjugated Ab. In vitro, A1mcMMAF exhibited receptor-density dependent cytotoxic activity against a panel of 5T4 + cell lines with an IC50 of 5 ng Ab/ml against high 5T4 expressing cells. A1mcMMAF exerts no inhibitory effect on 5T4 negative cells demonstrating it does not exert off-target activity in vitro. In vivo, A1mcMMAF causes long-term (100 days) tumor regressions at 10 mg Ab/kg in MDAMB435/5T4 tumor xenografts (Q4d×2, high 5T4 expression by IHC) and in the NSCLC patient-derived xenograft 37622A model (Q4d×4, high 5T4 expression by IHC though only a portion of the cells express 5T4). Significant anti-tumor activity was demonstrated at 1 mg/kg. Against the low 5T4 expressing H1975 tumor xenograft, A1mcMMAF causes potent tumor regression in spite of its lack of in vitro inhibitory activity against this cell line. Bioimaging of the H1975 tumor demonstrated good penetration of Alexa 750 labeled A1 Ab into the xenograft. A1mcMMAF exhibited no overt toxicities when administered at 10 mg Ab/kg/cycle × 2 to cynomolgus macques and possessed a half-life of 4 to 7 days. The observation that A1mcMMAF regresses H1975 tumor xenografts while demonstrating poor activity against H1975 in vitro demonstrates the attractiveness of targeting 5T4 in the tumor environment and may be explained by 5T4's principal association with proliferating cells in the tumor. It is also interesting that A1mcMMAF was able to completely regress 37622A xenografts since this patient-derived xenograft presents heterogeneous 5T4 expression, i.e. not all cells demonstrate detectable levels of 5T4.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A215.

Delineation of a cellular hierarchy in lung cancer reveals an oncofetal antigen expressed on tumor-initiating cells

Poorly differentiated tumors in non-small cell lung cancer (NSCLC) have been associated with shorter patient survival and shorter time to recurrence following treatment. Here, we integrate multiple experimental models with clinicopathologic analysis of patient tumors to delineate a cellular hierarchy in NSCLC. We show that the oncofetal protein 5T4 is expressed on tumor-initiating cells and associated with worse clinical outcome in NSCLC. Coexpression of 5T4 and factors involved in the epithelial-to-mesenchymal transition were observed in undifferentiated but not in differentiated tumor cells. Despite heterogeneous expression of 5T4 in NSCLC patient-derived xenografts, treatment with an anti-5T4 antibody-drug conjugate resulted in complete and sustained tumor regression. Thus, the aggressive growth of heterogeneous solid tumors can be blocked by therapeutic agents that target a subpopulation of cells near the top of the cellular hierarchy.

Tumour-initiating cells: challenges and opportunities for anticancer drug discovery

The hypothesis that cancer is driven by tumour-initiating cells (popularly known as cancer stem cells) has recently attracted a great deal of attention, owing to the promise of a novel cellular target for the treatment of haematopoietic and solid malignancies. Furthermore, it seems that tumour-initiating cells might be resistant to many conventional cancer therapies, which might explain the limitations of these agents in curing human malignancies. Although much work is still needed to identify and characterize tumour-initiating cells, efforts are now being directed towards identifying therapeutic strategies that could target these cells. This Review considers recent advances in the cancer stem cell field, focusing on the challenges and opportunities for anticancer drug discovery.

Hyperconserved CpG domains underlie Polycomb-binding sites

Comparative genomics of CpG dinucleotides, which are targets of DNA methyltransferases in vertebrate genomes, has been constrained by their evolutionary instability and by the effect of methylation on their mutation rates. We compared the human and chimpanzee genomes to identify DNA sequence signatures correlated with rates of mutation at CpG dinucleotides. The new signatures were used to develop robust comparative genomics of CpG dinucleotides in heterogeneous regions and to identify genomic domains that have anomalous CpG divergence rates. The data showed that there are approximately 200 genomic regions where CpG distributions are far more conserved than predicted. These hyperconserved CpG domains largely coincide with domains bound by Polycomb repressive complex 2 in undifferentiated human embryonic stem cells and are almost exclusively present near genes whose products are involved in the regulation of embryonic development. Several domains were experimentally shown to be unmethylated at different developmental stages. These data indicate that particular evolutionary patterns and distinct sequence properties on scales much larger than standard transcription factor-binding sites may play an important role in Polycomb recruitment and transcriptional regulation of key developmental genes.

Biological functions of DNA methyltransferase 1 require its methyltransferase activity

DNA methyltransferase 1 (DNMT1) has been reported to interact with a wide variety of factors and to contain intrinsic transcriptional repressor activity. When a conservative point mutation was introduced at the key catalytic residue, mutant DNMT1 failed to rescue any of the phenotypes of Dnmt1-null embryonic stem (ES) cells, which indicated that the biological functions of DNMT1 are exerted through the methylation of DNA. ES cells that expressed the mutant protein did not survive differentiation. Intracisternal A-particle family retrotransposons were no longer methylated and were transcribed at high levels. The proper localization of DNMT1 depended on normal genomic methylation, and we discuss the implications of this finding for epigenetic dysregulation in cancer.

The decatenation checkpoint

The decatenation checkpoint delays entry into mitosis until the chromosomes have been disentangled. Deficiency in or bypass of the decatenation checkpoint can cause chromosome breakage and nondisjunction during mitosis, which results in aneuploidy and chromosome rearrangements in the daughter cells. A deficiency in the decatenation checkpoint has been reported in lung and bladder cancer cell lines and may contribute to the accumulation of chromosome aberrations that commonly occur during tumour progression. A checkpoint deficiency has also been documented in cultured stem and progenitor cells, and cancer stem cells are likely to be derived from stem and progenitor cells that lack an effective decatenation checkpoint. An inefficient decatenation checkpoint is likely to be a source of the chromosome aberrations that are common features of most tumours, but an inefficient decatenation checkpoint in cancer stem cells could also provide a potential target for chemotherapy.

Analysis of Protein Interactions In Vivo with Fluorescence Resonance Energy Transfer (FRET)

INTRODUCTIONThe protocol presented here focuses on a standard microscope-based fluorescence resonance energy transfer (FRET) assay for budding yeast, but many aspects are applicable to other systems and assays. It provides information on experimental design and data analysis procedures that can be extended to FRET experiments in any system, including mammalian cell lines and other model organisms. The steps describing strain construction can be applied to studies in yeast using fluorimetry instead of microscopy. Although the data acquisition and analysis described in this protocol can be a daunting task, careful quantitative analysis is absolutely necessary because of a high degree of cross-talk and many unknown parameters that vary from cell to cell.

Decatenation checkpoint deficiency in stem and progenitor cells

The decatenation checkpoint normally delays entry into mitosis until chromosomes have been disentangled through the action of topoisomerase II. We have found that the decatenation checkpoint is highly inefficient in mouse embryonic stem cells, mouse neural progenitor cells, and human CD34+ hematopoietic progenitor cells. Checkpoint efficiency increased when embryonic stem cells were induced to differentiate, which suggests that the deficiency is a feature of the undifferentiated state. Embryonic stem cells completed cell division in the presence of entangled chromosomes, which resulted in severe aneuploidy in the daughter cells. The decatenation checkpoint deficiency is likely to increase the rates of chromosome aberrations in progenitor cells, stem cells, and cancer stem cells.

In situ analysis of spatial relationships between proteins of the nuclear pore complex

Macromolecular transport between the nucleus and cytoplasm occurs through the nuclear pore complexes (NPCs). The NPC in the budding yeast Saccharomyces cerevisiae is a 60-MDa structure embedded in the nuclear envelope and composed of ~30 proteins, termed nucleoporins or nups. Here we present a large-scale analysis of spatial relationships between nucleoporins using fluorescence resonance energy transfer (FRET) in living yeast cells. Energy transfer was measured in a panel of strains, each of which coexpresses the enhanced cyan and yellow fluorescent proteins as fusions to distinct nucleoporins. With this approach, we have determined 13 nucleoporin pairs yielding FRET signals. Independent experiments are consistent with the FRET results: Nup120 localization is perturbed in the nic96-1 mutant, as is Nup82 localization in the nup116Delta mutant. To better understand the spatial relationship represented by an in vivo FRET signal, we have investigated the requirements of these signals. We demonstrate that in one case FRET signal is lost upon insertion of a short spacer between the nucleoporin and its enhanced yellow fluorescent protein label. We also show that the Nup120 FRET signals depend on whether the fluorescent moiety is fused to the N- or C-terminus of Nup120. Combined with existing data on NPC structure, the FRET pairs identified in this study allow us to propose a refined molecular model of the NPC. We suggest that the approach may serve as a prototype for the in situ study of other large macromolecular complexes.

The genome-wide localization of Rsc9, a component of the RSC chromatin-remodeling complex, changes in response to stress

The cellular response to environmental changes includes widespread modifications in gene expression. Here we report the identification and characterization of Rsc9, a member of the RSC chromatin-remodeling complex in yeast. The genome-wide localization of Rsc9 indicated a relationship between genes targeted by Rsc9 and genes regulated by stress; treatment with hydrogen peroxide or rapamycin, which inhibits TOR signaling, resulted in genome-wide changes in Rsc9 occupancy. We further show that Rsc9 is involved in both repression and activation of mRNAs regulated by TOR as well as the synthesis of rRNA. Our results illustrate the response of a chromatin-remodeling factor to signaling cascades and suggest that changes in the activity of chromatin-remodeling factors are reflected in changes in their localization in the genome.

Mapping interactions between nuclear transport factors in living cells reveals pathways through the nuclear pore complex

The interactions between transport receptors and proteins of the nuclear pore complex (NPC) are fundamental to understanding nucleocytoplasmic transport. In order to delineate the path that a particular transport receptor takes through the NPC, we have employed fluorescence resonance energy transfer (FRET) between enhanced cyan and yellow fluorescent proteins (ECFP, EYFP) in living cells. A panel of yeast strains expressing functional receptor--ECFP and nucleoporin--EYFP fusions has been analyzed with a FRET assay. With this approach, we define points of contact in the NPC for the related importin Pse1/Kap121 and exportin Msn5. These data demonstrate the utility of FRET in mapping dynamic protein interactions in a genetic system. Furthermore, the data indicate that an importin and exportin have overlapping pathways through the NPC.

Interactions between a nuclear transporter and a subset of nuclear pore complex proteins depend on Ran GTPase

Proteins to be transported into the nucleus are recognized by members of the importin-karyopherin nuclear transport receptor family. After docking at the nuclear pore complex (NPC), the cargo-receptor complex moves through the aqueous pore channel. Once cargo is released, the importin then moves back through the channel for new rounds of transport. Thus, importin and exportin, another member of this family involved in export, are thought to continuously shuttle between the nuclear interior and the cytoplasm. In order to understand how nuclear transporters traverse the NPC, we constructed functional protein fusions between several members of the yeast importin family, including Pse1p, Sxm1p, Xpo1p, and Kap95p, and the green fluorescent protein (GFP). Complexes containing nuclear transporters were isolated by using highly specific anti-GFP antibodies. Pse1-GFP was studied in the most detail. Pse1-GFP is in a complex with importin-alpha and -beta (Srp1p and Kap95p in yeast cells) that is sensitive to the nucleotide-bound state of the Ran GTPase. In addition, Pse1p associates with the nucleoporins Nsp1p, Nup159p, and Nup116p, while Sxm1p, Xpo1p, and Kap95p show different patterns of interaction with nucleoporins. Association of Pse1p with nucleoporins also depends on the nucleotide-bound state of Ran; when Ran is in the GTP-bound state, the nucleoporin association is lost. A mutant form of Pse1p that does not bind Ran also fails to interact with nucleoporins. These data indicate that transport receptors such as Pse1p interact in a Ran-dependent manner with certain nucleoporins. These nucleoporins may represent major docking sites for Pse1p as it moves in or out of the nucleus via the NPC.