Preclinical Evaluation of IMGC936, a Next Generation Maytansinoid-based Antibody-drug Conjugate Targeting ADAM9-expressing Tumors

A disintegrin and metalloprotease (ADAM) 9 is a member of the ADAM family of multifunctional, multidomain type 1 transmembrane proteins. ADAM9 is overexpressed in many cancers, including non-small cell lung, pancreatic, gastric, breast, ovarian, and colorectal cancer, but exhibits limited expression in normal tissues. A target-unbiased discovery platform based on intact tumor and progenitor cell immunizations, followed by an immunohistochemistry screen, led to the identification of anti-ADAM9 antibodies with selective tumor-versus-normal tissue binding. Subsequent analysis revealed anti-ADAM9 antibodies were efficiently internalized and processed by tumor cells making ADAM9 an attractive target for antibody-drug conjugate development. Here, we describe the preclinical evaluation of IMGC936, a novel antibody-drug conjugate targeted against ADAM9. IMGC936 is comprised of a high-affinity humanized antibody site-specifically conjugated to DM21-C, a next-generation linker-payload that combines a maytansinoid microtubule-disrupting payload with a stable tripeptide linker, at a drug antibody ratio of approximately 2.0. Additionally, the YTE mutation (M252Y/S254T/T256E) was introduced into the CH2 domain of the antibody Fc to maximize in vivo plasma half-life and exposure. IMGC936 exhibited cytotoxicity toward ADAM9-positive human tumor cell lines, as well as bystander killing, potent antitumor activity in human cell line-derived xenograft and patient-derived xenograft tumor models, and an acceptable safety profile in cynomolgus monkeys with favorable pharmacokinetic properties. Our preclinical data provide a strong scientific rationale for the further development of IMGC936 as a therapeutic candidate for the treatment of ADAM9-positive cancers. A first-in-human study of IMGC936 in patients with advanced solid tumors has been initiated (NCT04622774).

Abstract 2890: IMGN151 - A next generation folate receptor alpha targeting antibody drug conjugate active against tumors with low, medium and high receptor expression

Folate Receptor alpha (FRα) is an attractive antibody drug conjugate (ADC) target due to its over expression in multiple epithelial malignancies including ovarian, endometrial, triple negative breast, and non-small cell lung cancer, with limited expression on normal tissues. IMGN853 (i.e., mirvetuximab soravtansine and M9346A-sulfo-SPDB-DM4), a FRα targeting ADC, is currently in phase III (MIRASOL) clinical evaluation as monotherapy in patients with platinum-resistant epithelial ovarian cancer with high levels of FRα expression. The MIRASOL study builds on the results from the prior randomized study, FORWARD I, which demonstrated that improved outcomes with IMGN853 correlated with FRα expression, with the strongest treatment effects for all efficacy endpoints in ovarian cancer patients with FRα-high disease (Moore, ESMO 2019).

In order to address the unmet needs of additional patient populations, we sought to develop a next generation FRα-targeting ADC active against tumors with a broad range of FRα expression.

Development of a new molecular entity with the desired antitumor properties included optimization of the antibody format and the linker-payload. The resulting lead ADC denoted IMGN151 comprises an asymmetric, bivalent, biparatopic antibody targeting two independent epitopes of FRα, linked to the highly potent maytansinoid derivative DM21 via a stable cleavable peptide linker. The average drug per antibody ratio is 3.5.

The binding, internalization and processing of the biparatopic IMGN151 and the parent monospecific antibodies were compared using 3H-antibodies. In tumor cells with medium (JHOS4) and high (KB) FRα expression the biparatopic antibody boosted antibody binding events and processing by 100% and 170%, respectively.

The plasma stability of IMGN151 was tested in a cynomolgus monkey pharmacokinetic study. The stable linker increased ADC half-life by 60 hours and conjugate exposure in vivo by 40%, as compared to IMGN853.

IMGN151 activity was characterized against cell lines and xenograft models with a wide range of FRα expression and compared to IMGN853. In in vitro studies, both ADCs had similar activity against FRα-high KB cells; IMGN151 was up to 200 times more active against four FRα-medium cell lines. IMGN151 had also notably stronger bystander killing activity in a mixed culture of target-positive and negative cells. In vivo IMGN151 induced complete tumor regressions of human tumor xenograft models with high (KB, H-score of 300), medium (Igrov-1 and Ishikawa, H-score of 140 and 100, respectively) and low (Ov-90, H-score of 30) FRα expression. All tested doses were well tolerated with no body weight loss observed.

With a novel biparatopic antibody and linker payload design, IMGN151 has shown potent antitumor activity against ovarian cancer models with a broad range of FRα expression, which warrants further development into the clinic for patients with tumors expressing FRα at a wide range of levels.

Citation Format: Olga Ab, Laura M. Bartle, Leanne Lanieri, Jose F. Ponte, Qifeng Qiu, Surina Sikka, Juliet A. Costoplus, Wayne Deats, Nicholas C. Yoder, Wayne C. Widdison, Katherine Mucciarone, Kate Selvitelli, Ying Chen, Neeraj Kohli, Thomas Chittenden, Richard Gregory, Yulius Setiady, Eric H. Westin. IMGN151 - A next generation folate receptor alpha targeting antibody drug conjugate active against tumors with low, medium and high receptor expression [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 2890.

Site-Specific Conjugation of the Indolinobenzodiazepine DGN549 to Antibodies Affords Antibody-Drug Conjugates with an Improved Therapeutic Index as Compared with Lysine Conjugation

Antibody-drug conjugates have elicited great interest recently as targeted chemotherapies for cancer. Recent preclinical and clinical data have continued to raise questions about optimizing the design of these complex therapeutics. Biochemical methods for site-specific antibody conjugation have been a design feature of recent clinical ADCs, and preclinical reports suggest that site-specifically conjugated ADCs generically offer improved therapeutic indices (i.e., the fold difference between efficacious and maximum tolerated doses). Here we present the results of a systematic preclinical comparison of ADCs embodying the DNA-alkylating linker-payload DGN549 generated with both heterogeneous lysine-directed and site-specific cysteine-directed conjugation chemistries. Importantly, the catabolites generated by each ADC are the same regardless of the conjugation format. In two different model systems evaluated, the site-specific ADC showed a therapeutic index benefit. However, the therapeutic index benefit is different in each case: both show evidence of improved tolerability, though with different magnitudes, and in one case significant efficacy improvement is also observed. These results support our contention that conjugation chemistry of ADCs is best evaluated in the context of a particular antibody, target, and linker-payload, and ideally across multiple disease models.

Design, synthesis and evaluation of novel, potent DNA alkylating agents and their antibody-drug conjugates (ADCs)

Antibody-drug conjugates (ADCs) incorporating potent indolinobenzodiazepine (IGN) DNA alkylators as the cytotoxic payload are currently undergoing clinical evaluation. The optimized design of these payloads consists of an unsymmetrical dimer possessing both an imine and an amine effectively eliminating DNA crosslinking and demonstrating improved tolerability in mice. Here we present an alternate approach to generating DNA alkylating ADCs by linking the IGN monomer with a biaryl system which has a high DNA binding affinity to potentially enhance tolerability. These BIA ADCs were found to be highly cytotoxic in vitro and demonstrated potent antitumor activity in vivo.

Effect of Linker Stereochemistry on the Activity of Indolinobenzodiazepine Containing Antibody-Drug Conjugates (ADCs)

Antibody-drug conjugates (ADCs) that incorporate potent indolinobenzodiazepine DNA alkylators as the payload component are currently undergoing clinical evaluation. In one ADC design, the payload molecules are linked to the antibody through a peptidase-labile l-Ala-l-Ala linker. In order to determine the role of amino acid stereochemistry on antitumor activity and tolerability, we incorporated l- and d-alanyl groups in the dipeptide, synthesized all four diastereomers, and prepared and tested the corresponding ADCs. Results of our preclinical evaluation showed that the l-Ala-l-Ala configuration provided the ADC with the highest therapeutic index (antitumor activity vs toxicity).

A Case Study Comparing Heterogeneous Lysine- and Site-Specific Cysteine-Conjugated Maytansinoid Antibody-Drug Conjugates (ADCs) Illustrates the Benefits of Lysine Conjugation

Antibody-drug conjugates are an emerging class of cancer therapeutics constructed from monoclonal antibodies conjugated with small molecule effectors. First-generation molecules of this class often employed heterogeneous conjugation chemistry, but many site-specifically conjugated ADCs have been described recently. Here, we undertake a systematic comparison of ADCs made with the same antibody and the same macrocyclic maytansinoid effector but conjugated either heterogeneously at lysine residues or site-specifically at cysteine residues. Characterization of these ADCs in vitro reveals generally similar properties, including a similar catabolite profile, a key element in making a meaningful comparison of conjugation chemistries. In a mouse model of cervical cancer, the lysine-conjugated ADC affords greater efficacy on a molar payload basis. Rather than making general conclusions about ADCs conjugated by a particular chemistry, we interpret these results as highlighting the complexity of ADCs and the interplay between payload class, linker chemistry, target antigen, and other variables that determine efficacy in a given setting.

Abstract 229: Utilizing a mouse cross-reactive model system to better understand antibody-drug conjugate pharmacokinetics, biodistribution and efficacy

Antibody-drug conjugates (ADCs) are designed to deliver a potent cytotoxic payload directly to tumors, thus limiting exposure in normal tissues. However, target antigen expression on normal tissues can lead to lower systemic ADC exposures, resulting in sub-efficacious concentrations at the tumor site as well as heterogeneous distribution within tumors. Traditional preclinical efficacy studies performed in rodent models using ADCs with non-cross-reactive antibodies have been of limited translational relevance, and a better understanding of the factors that impact ADC dose and activity remains to be elucidated. To examine relationships between variables that could influence ADC efficacy, we generated a cross-reactive model system that utilized a chimeric anti-murine folate receptor α (FRα) antibody (designated rmFR1-12) that binds both mouse and human FRα, and can be conjugated to either maytansinoid (DM) or indolinobenzodiazepine (IGN) payloads. This model system was predicted to have substantial target-mediated drug disposition (TMDD) due to normal tissue expression of FRα. An rmFR1-12-s-SPDB-DM4 ADC was made using tritium-labeled DM4 and administered to tumor-bearing mice in order to assess ADC pharmacokinetics (PK), biodistribution, and efficacy. This approach allowed tracking of the ADC at multiple levels – whole animal, organ, tumor, and cell. Studies were undertaken that assessed the impact of xenograft antigen expression, ADC dose, and ADC drug-to-antibody ratio (DAR) on the PK, biodistribution, and efficacy of the rmFR1-12-s-SPDB-DM4 conjugate. The results showed that TMDD significantly impacted the PK, biodistribution, and activity of the conjugate relative to a non-cross-reactive ADC, with lower ADC doses being more severely impacted than higher doses. Antigen expression positively correlated with local ADC exposure and efficacy. Decreasing the DAR (by co-dosing naked antibody with the ADC) increased systemic exposure. A positive correlation between systemic exposure and dose of naked antibody was observed. Factors that impacted local exposure included: the type of tissue (normal vs tumor), presence or absence of FRα expression on normal tissue, and dose of naked antibody. Of note, in the naked antibody plus ADC study, efficacy did not correlate with local exposure - suggesting that ADC distribution within tumors is of similar importance as the amount of ADC delivered. Overall, these findings underscore the importance of accounting for site, and extent of, normal tissue target expression with respect to ADC PK/PD, and the data generated from these studies are currently being used to build a multiscale physiologically based PK model of a cross-reactive ADC.

Citation Format: Leanne Lanieri, Rassol Laleau, Bahar Matin, Jenny Lee, Steven Boule, Paulin Salomon, Luke Harris, Michael Miller, Nicholas C. Yoder, Yulius Setiady, Neeraj Kohli, Thomas A. Keating, Jan Pinkas, Richard Gregory. Utilizing a mouse cross-reactive model system to better understand antibody-drug conjugate pharmacokinetics, biodistribution and efficacy [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 229.

Abstract 747: A new class of DNA alkylating indolino-benzodiazepine agents (BIAs) linked with a DNA binding moiety for use with antibody-drug conjugates (ADCs)

We have previously disclosed antibody-drug conjugates (ADCs) that incorporate our highly potent novel DNA alkylating indolino-benzodiazepine (termed IGN) pseudo dimer. ADCs of these DNA alkylating IGNs were found preclinically to demonstrate better tolerability and an improved overall therapeutic index (TI) compared with those of DNA crosslinking IGNs (Miller, et al., AACR 2017 #53). In our ongoing effort to further explore DNA alkylating effector molecules for ADCs, we disclose here a new class of IGNs (termed BIAs) in which one IGN monomer subunit is connected to a bis-aryl moiety with affinity for the DNA binding pocket. Our initial structure activity relationship (SAR) studies around a series of BIAs identified a set of scaffolds that met our requirement for high in vitro potency. These scaffolds were subsequently modified to incorporate functionalities that allow linkage to an antibody using various linker chemistries. These linkable BIAs displayed high in vitro potency across a panel of cell lines, indicating that the incorporation of a linker was not detrimental to their overall potency. Furthermore, we found that the potency of these linkable BIAs could be modulated by careful selection of substituents on the bis-aryl moiety providing enhanced binding affinity with DNA. Upon identification of lead BIA molecules, conjugates with a folate receptor-α (FRα)-binding antibody were prepared. These ADCs displayed potent, antigen-specific in vitro activity across a panel of FRα-expressing cell lines. In vivo, these ADCs demonstrated potent efficacy in xenograft models at doses well below the maximum tolerated dose. In light of these findings, BIA ADCs represent a promising new class of DNA alkylating effector molecules for use in the development of ADCs.

Citation Format: Michael L. Miller, Emily E. Reid, Katie E. Archer, Manami Shizuka, Molly A. McShea, Erin K. Maloney, Olga Ab, Leanne Lanieri, Alan J. Wilhelm, Jose F. Ponte, Nicholas C. Yoder, Ravi V. Chari. A new class of DNA alkylating indolino-benzodiazepine agents (BIAs) linked with a DNA binding moiety for use with antibody-drug conjugates (ADCs) [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 747.

Abstract 75: Comparison of site-specific and lysine-linked indolino-benzodiazepine antibody-drug conjugates (ADCs)

ADCs are a promising modality for cancer therapy enabled by chemical conjugation of potent cytotoxic compounds to monoclonal antibodies. While many ADCs in clinical evaluation employ heterogeneous conjugation chemistries where the payload is linked through lysine or endogenous cysteine residues, there has recently been considerable interest in site-specific conjugation. ADCs prepared using site-specific methods are believed to have a wider therapeutic index compared to heterogeneous ADCs. We have previously shown that site-specific ADCs incorporating the maytansinoid-based tubulin inhibitor DM1 were less efficacious in vivo when compared to analogous lysine-linked conjugates (Yoder et al AACR 2015 Abstract #545). More recently, we presented results from the evaluation of 2.5-3.0 drug-to-antibody ratio (DAR) heterogeneous lysine-linked and 2 DAR site-specific ADCs using antibodies with engineered cysteines at position 442 in the heavy chains (known as CYSMABTM). These ADCs used the peptide-linked indolino-benzodiazepine DNA alkylator DGN549 (also known as IGN-P1) as the effector. Unlike the DM1 case, site-specific DGN549 ADCs were at least as active in vivo when compared to lysine-linked ADCs (Yoder et al AACR 2016 Abstract #2960). We have made further pharmacological comparisons between CYSMAB and lysine-linked DGN549 ADCs at matched payload doses using two different antibodies targeting distinct cell surface receptors. In the case of mAb1, CYSMAB and lysine-linked ADCs were comparably active. For mAb2, the CYSMAB ADC was more active than the lysine-linked ADC in some models and similarly active in others. The mAb1 CYSMAB ADC exhibited a significantly higher maximum tolerated dose (MTD) compared to the lysine-linked ADC. In contrast, the MTDs of the mAb2 ADCs were similar. However, the mAb2 CYSMAB conjugate was better tolerated in terms of median lethal dose. The mechanism for the improved tolerability of the mAb1 CYSMAB conjugate is not apparent. In an effort to understand whether it is a consequence of conjugation chemistry or DAR, we compared the tolerability of the mAb1 CYSMAB conjugate to that of a ~4 DAR site-specific analog and found that both factors contribute. To determine if our observations can be rationalized in terms of in vivo disposition, we compared the pharmacokinetics of mAb1 CYSMAB and lysine-linked ADCs at matched antibody doses. Intriguingly, the CYSMAB ADC showed slightly greater exposure. These results, along with our previous work on site-specific DM1 ADCs, indicate that in some cases site-specific conjugation can lead to improved efficacy and tolerability. However, generalizations cannot be made across different combinations of antibody, linker, and payload. We conclude that the advantages and disadvantages of site-specific conjugation should be carefully considered for every candidate ADC.

Citation Format: Chen Bai, Nicholas C. Yoder, Alan Wilhelm, Sharlene Adams, Kathleen Whiteman, Jenny Lee, Katie O'Callaghan, Erin Maloney, Manami Shizuka, Yelena Kovtun, Thomas A. Keating. Comparison of site-specific and lysine-linked indolino-benzodiazepine antibody-drug conjugates (ADCs) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 75. doi:10.1158/1538-7445.AM2017-75

Abstract 37: Novel antibody-drug conjugates targeting ADAM9-expressing solid tumors demonstrate potent preclinical activity

ADAM9, also known as MDC9 or meltrin-γ, is a member of the ADAM (a disintegrin and metalloproteinase) family of proteases, which have been implicated in cytokine and growth factor shedding, and cell migration. Dysregulation of ADAM9 has been implicated in tumor progression and metastasis, as well as pathological neovascularization. ADAM9 overexpression has been shown to correlate with poor prognosis in prostate, renal, and pancreatic cancers. Using an immunization approach in which antibodies were raised to fetal progenitor and stem-like cancer cell lines followed by screening on tumor and normal tissues, we identified ADAM9 as a promising cell surface tumor target. FFPE-IHC expression analysis revealed that ADAM9 is overexpressed in multiple solid tumor indications relative to corresponding normal tissues. The overexpression of ADAM9 in tumors coupled with its restricted expression in normal tissues make ADAM9 an attractive target for antibody-drug conjugate (ADC) therapy.

Here, we describe two ADCs both of which are based on a high affinity anti-ADAM9 antibody to selectively target ADAM9-expressing tumors. The first ADC utilizes the maytansine-derived microtubule disruptor, DM4, linked via a hindered disulfide hydrophilic linker (sulfo-SPDB). The second ADC exploits an ultra-potent DNA alkylating payload, DGN549, which is conjugated to two engineered cysteines via a peptide linker. Both conjugates bound with similar subnanomolar affinity to ADAM9-expressing cells. In vitro cytotoxicity studies showed that anti-ADAM9 ADCs can kill a broad panel of ADAM9-positve tumor cell lines, including lung, pancreatic, renal, prostate, and colon tumor cell lines. In particular, the anti-ADAM9-DGN549 conjugate was extremely potent with IC50 values ranging from 0.1 to 65 pM and was at least 2 logs more active than a non-targeting conjugate. Surprisingly, efficient in vitro cytotoxicity was observed at ADAM9 expression levels as low as a few thousand cell surface receptors per cell.

Consistent with their in vitro activity, both anti-ADAM9 ADCs displayed compelling anti-tumor activity in xenograft models. In a CaLu3 non-small cell lung cancer xenograft model, anti-ADAM9-DM4 induced tumor growth delay at a single 1.25 mg Ab/kg dose. In the same model, a single intravenous dose of 0.25 mg Ab/kg of the anti-ADAM9-DGN549 produced durable complete remissions in 8/8 mice. A non-targeting DGN549 ADC was inactive even when dosed at 10 times that of the anti-ADAM9 ADC, demonstrating that targeted delivery of DGN549 through ADAM9 binding is required for activity.

These data demonstrate that anti-ADAM9 ADCs exhibit antitumor activity against a broad panel of ADAM9-positive malignancies and cause durable remissions in preclinical models at doses expected to be clinically achievable. Anti-ADAM9 ADCs represent a promising therapeutic strategy to target a wide range of ADAM9-expressing tumors.

Citation Format: Stuart W. Hicks, Nicholas C. Yoder, Deryk Loo, Asli Muvaffak, Yinghui Zhou, Megan E. Fuller, Molly A. McShea, Marian Themeles, Katherine H. Mucciarone, Juniper A. Scribner, Bhaswati Barat, Thomas Sun, James Tamura, Francine Z. Chen, Kerry A. Donahue, Tom Chittenden. Novel antibody-drug conjugates targeting ADAM9-expressing solid tumors demonstrate potent preclinical activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 37. doi:10.1158/1538-7445.AM2017-37

Abstract 38: Target validation, antibody discovery and preclinical data supporting ADAM9 as an antibody-drug conjugate therapeutic target for solid tumors

Introduction: A target-unbiased approach based on intact cell immunizations with fetal progenitor cells and cancer stem cells, followed by an immunohistochemistry (IHC) screen for cancer-specific candidates, led to the identification of anti-ADAM9 (a disintegrin and metalloproteinase) mAbs with highly differential tumor-versus-normal tissue binding. ADAM9 is a cell surface protein over-expressed in multiple tumors, with a possible role in promotion and progression of cancer through multiple mechanisms, including modulation of adhesion and migration as well as processing of tumorigenic and pro-angiogenic factors. In this preclinical study, we performed target/mAb validation and evaluated the therapeutic potential of anti-ADAM9 antibody-drug conjugates (ADCs) toward ADAM9-expressing solid cancers.

Methods: IHC was performed with anti-ADAM9 mAbs to confirm and extend available data of human normal and tumor tissue expression. Epitope mapping studies were conducted to define epitope-specificity. mAbs were also screened to identify those that efficiently internalized into tumor cells. In vitro cellular processing studies were performed to further evaluate the mAbs as ADC candidates. Selected mAbs were converted to ADCs via chemical conjugation to potent anti-microtubule (DM4) or DNA alkylating (DGN549) agents; in vitro cytotoxicity studies were conducted with tumor cell lines representing human cancer types that overexpress ADAM9. A lead mAb was then selected for humanization and affinity maturation to yield a development candidate.

Results: Anti-ADAM9 mAbs exhibited strong reactivity toward the tumor epithelium of solid cancers, including pancreatic, kidney, prostate, bladder, breast, colon, lung, and ovarian cancer, but limited reactivity toward normal tissues. Anti-ADAM9 mAbs were efficiently internalized and processed by tumor cell lines, including lines with only modest ADAM9 expression. Anti-ADAM9 ADCs exhibited specific, dose-dependent cytotoxicity toward ADAM9-positive cancer cell lines in vitro, with IC50 values in the sub-nanomolar range. Humanization and affinity maturation of the lead mAb yielded a development candidate that retains potent antitumor activity toward ADAM9-positive tumor cell lines and equivalent, high affinity binding to both human and cynomolgus monkey ADAM9.

Conclusion: ADAM9 is a cell surface antigen that is over-expressed on a wide range of solid cancers. Anti-ADAM9 mAbs that were strongly reactive with representative tumors exhibited high affinity for the antigen and were efficiently internalized and processed by ADAM9-bearing tumor cells. Anti-ADAM9 ADCs demonstrated dose-dependent cytotoxicity in vitro toward a panel of ADAM9-positive tumor cell lines. Our findings demonstrate that an ADC targeting ADAM9 may serve as a potential therapeutic for ADAM9-expressing solid tumors.

Citation Format: Juniper A. Scribner, Bhaswati Barat, Stuart W. Hicks, Nicholas C. Yoder, Thomas Son, Lusiana Widjaja, Gundo Diedrich, Sergey Gorlatov, Jeff Hooley, Ann Easton, Peter Lung, Anushka De Costa, Francine Chen, Michael Chiechi, Pam Li, Monica Licea, Timothy E. Hotaling, Michael Spliedt, Valentina Ciccarone, Nadia Gantt, James Tamura, Megan E. Fuller, Molly McShea, Scott Koenig, Syd Johnson, Paul A. Moore, Ezio Bonvini, Deryk Loo. Target validation, antibody discovery and preclinical data supporting ADAM9 as an antibody-drug conjugate therapeutic target for solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 38. doi:10.1158/1538-7445.AM2017-38

Discovery and Optimization of HKT288, a Cadherin-6-Targeting ADC for the Treatment of Ovarian and Renal Cancers

Despite an improving therapeutic landscape, significant challenges remain in treating the majority of patients with advanced ovarian or renal cancer. We identified the cell-cell adhesion molecule cadherin-6 (CDH6) as a lineage gene having significant differential expression in ovarian and kidney cancers. HKT288 is an optimized CDH6-targeting DM4-based antibody-drug conjugate (ADC) developed for the treatment of these diseases. Our study provides mechanistic evidence supporting the importance of linker choice for optimal antitumor activity and highlights CDH6 as an antigen for biotherapeutic development. To more robustly predict patient benefit of targeting CDH6, we incorporate a population-based patient-derived xenograft (PDX) clinical trial (PCT) to capture the heterogeneity of response across an unselected cohort of 30 models-a novel preclinical approach in ADC development. HKT288 induces durable tumor regressions of ovarian and renal cancer models in vivo, including 40% of models on the PCT, and features a preclinical safety profile supportive of progression toward clinical evaluation.Significance: We identify CDH6 as a target for biotherapeutics development and demonstrate how an integrated pharmacology strategy that incorporates mechanistic pharmacodynamics and toxicology studies provides a rich dataset for optimizing the therapeutic format. We highlight how a population-based PDX clinical trial and retrospective biomarker analysis can provide correlates of activity and response to guide initial patient selection for first-in-human trials of HKT288. Cancer Discov; 7(9); 1030-45. ©2017 AACR.This article is highlighted in the In This Issue feature, p. 920.

Abstract 2960: Potent in vivo activity of site-specific indolino-benzodiazepine antibody-drug conjugates (ADCs) generated via engineered cysteine conjugation

ADCs are widely studied for cancer therapy, with numerous agents in preclinical and clinical development embodying a wide array of targets, linker chemistries, and cytotoxic effector classes. A fourth element of ADC design that has received much attention recently is the site of conjugation of the cytotoxic molecule to the antibody. Historically, lysine- or interchain cysteine-directed conjugation has been used, but site-specific chemistries have become increasingly popular. Our previous evaluation of site-specific and lysine-linked ADCs utilizing a tubulin-acting maytansinoid effector molecule found the lysine-linked version was more active in vivo (Yoder et al., AACR 2015 #645). Here we present a comparison of engineered cysteine site-specific and lysine-linked ADCs utilizing the previously described indolino-benzodiazepine (henceforth referred to as IGN) effector IGN-P1 (Miller et al., AACR 2015 #652) which is designed to undergo proteolytic cleavage upon cell uptake to release a potently cytotoxic DNA alkylator.

We show that HC-S442C mutants of human IgG1 can be conjugated via maleimide chemistry to IGN-P1 to give stable, potent, and homogeneous ADCs with drug to antibody ratio (DAR) of 2. The in vitro potency of engineered-cysteine IGN-P1 ADCs is largely dependent on the DAR of the ADC, although some difference is observed between HC-S442C and other cysteine mutants used for conjugation.

Pharmacokinetic study of C442 maleimide conjugates suggests that the chemical linkage between effector and antibody is stable upon administration in mice. Further, and in contrast to our previous observations utilizing maytansinoid ADCs, the site-specific and Lys-linked IGN-P1 ADCs showed comparable efficacy in vivo on a molar drug basis. This effect was observed across two different antibodies targeting two different cell surface antigens. These results suggest that, in certain cases, site-specific conjugation chemistry can offer comparable activity to heterogeneous conjugation at well-tolerated doses.

Citation Format: Nicholas C. Yoder, Chen Bai, Alan Wilhelm, Erin K. Maloney, Olga Ab, Emily E. Reid, Manami Shizuka, Daniel Tavares, Rassol Laleau, Xiuxia Sun, Megan E. Bogalhas, Lintao Wang, Jan Pinkas, Michael L. Miller, Ravi Chari, Thomas A. Keating. Potent in vivo activity of site-specific indolino-benzodiazepine antibody-drug conjugates (ADCs) generated via engineered cysteine conjugation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2960.

Abstract 2959: Peptide-linked indolino-benzodiazepine DNA-alkylating agents for use in antibody-drug conjugates (ADCs)

As part of our efforts to expand the available toolbox of cytotoxic payloads for use in antibody-drug conjugates (ADCs), we have recently described the development of the mono-imine-containing indolino-benzodiazepine dimer, IGN-P1 (Miller, et al., AACR 2014 #652). IGN-P1 was conjugated via a protease-cleavable peptidic L-alanine-L-alanine linker to a folate receptor α (FRα)-binding antibody, and an EGFR-binding antibody. The resulting ADCs demonstrated high in vitro potency (IC50 ∼4-100 pM) and specificity towards several cancer cell lines. In vivo, anti-FRα-IGN-P1 induced complete regressions in NCI-H2110 non-small cell lung cancer xenografts following a single dose of 3 μg/kg (linked payload dose, equivalent to 0.18 mg/kg Ab dose).

Here we describe the structure-activity relationship of a number of peptide-linked IGN ADCs, leading to the selection of Ab-IGN-P1 for further advancement. The cytotoxic activities of these peptide-IGN conjugates were evaluated in vitro in cancer cell lines with both high and low target antigen expression. We also assessed bystander activity and identified in vitro and in vivo catabolites. We found that the stereochemistry of the peptide linker was crucial for bystander activity and in vivo efficacy. The IGN-P1 ADC exhibited strong bystander activity, which we believe is dictating the strong antitumor activity in vivo. Treatment of low to moderate antigen-expressing models with Ab-IGN-P1 further resulted in tumor regressions at doses as low as 3-10 μg/kg linked payload.

These data underscore the potential therapeutic benefit of highly active and specific IGN-P1 conjugates, even for the challenging subset of patients with solid tumors where the target antigen is expressed at lower levels.

Citation Format: Manami Shizuka, Alan Wilhelm, Katie Archer, Emily Reid, Nicholas C. Yoder, Chen Bai, Nathan Fishkin, Luke Harris, Erin Maloney, Erica Hong, Rui Wu, Olga Ab, Kate Lai, Surina Sikka, Shan Jin, Jan Pinkas, Ravi Chari, Michael L. Miller. Peptide-linked indolino-benzodiazepine DNA-alkylating agents for use in antibody-drug conjugates (ADCs). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2959.

Understanding How the Stability of the Thiol-Maleimide Linkage Impacts the Pharmacokinetics of Lysine-Linked Antibody-Maytansinoid Conjugates

Antibody-drug conjugates (ADCs) have become a widely investigated modality for cancer therapy, in part due to the clinical findings with ado-trastuzumab emtansine (Kadcyla). Ado-trastuzumab emtansine utilizes the Ab-SMCC-DM1 format, in which the thiol-functionalized maytansinoid cytotoxic agent, DM1, is linked to the antibody (Ab) via the maleimide moiety of the heterobifunctional SMCC linker. The pharmacokinetic (PK) data for ado-trastuzumab emtansine point to a faster clearance for the ADC than for total antibody. Cytotoxic agent release in plasma has been reported with nonmaytansinoid, cysteine-linked ADCs via thiol-maleimide exchange, for example, brentuximab vedotin. For Ab-SMCC-DM1 ADCs, however, the main catabolite reported is lysine-SMCC-DM1, the expected product of intracellular antibody proteolysis. To understand these observations better, we conducted a series of studies to examine the stability of the thiol-maleimide linkage, utilizing the EGFR-targeting conjugate, J2898A-SMCC-DM1, and comparing it with a control ADC made with a noncleavable linker that lacked a thiol-maleimide adduct (J2898A-(CH2)3-DM). We employed radiolabeled ADCs to directly measure both the antibody and the ADC components in plasma. The PK properties of the conjugated antibody moiety of the two conjugates, J2898A-SMCC-DM1 and J2898A-(CH2)3-DM (each with an average of 3.0 to 3.4 maytansinoid molecules per antibody), appear to be similar to that of the unconjugated antibody. Clearance values of the intact conjugates were slightly faster than those of the Ab components. Furthermore, J2898A-SMCC-DM1 clears slightly faster than J2898A-(CH2)3-DM, suggesting that there is a fraction of maytansinoid loss from the SMCC-DM1 ADC, possibly through a thiol-maleimide dependent mechanism. Experiments on ex vivo stability confirm that some loss of maytansinoid from Ab-SMCC-DM1 conjugates can occur via thiol elimination, but at a slower rate than the corresponding rate of loss reported for thiol-maleimide links formed at thiols derived by reduction of endogenous cysteine residues in antibodies, consistent with expected differences in thiol-maleimide stability related to thiol pKa. These findings inform the design strategy for future ADCs.

Microscale screening of antibody libraries as maytansinoid antibody-drug conjugates

Antibody-drug conjugates (ADCs) are of great interest as targeted cancer therapeutics. Preparation of ADCs for early stage screening is constrained by purification and biochemical analysis techniques that necessitate burdensome quantities of antibody. Here we describe a method, developed for the maytansinoid class of ADCs, enabling parallel conjugation of antibodies in 96-well format. The method utilizes ∼ 100 µg of antibody per well and requires <5 µg of ADC for characterization. We demonstrate the capabilities of this system using model antibodies. We also provide multiple examples applying this method to early-stage screening of maytansinoid ADCs. The method can greatly increase the throughput with which candidate ADCs can be screened in cell-based assays, and may be more generally applicable to high-throughput preparation and screening of different types of protein conjugates.

Abstract 4531: Effects of drug load on therapeutic index for antibody-maytansinoid conjugates

Antibody-drug conjugates (ADCs) are being actively pursued as a new modality to treat cancer following the regulatory approval of Adcetris@ and Kadcyla@. ADCs consist of a cytotoxic agent, or drug, conjugated to a targeting antibody (Ab) through a linker. The two approved ADCs (and most ADCs now in the clinic) are heterogeneous conjugates with an average molar drug to Ab ratio (DAR) of 3-4 (potentially ranging from 0-8 for individual molecules). To understand the effects of different DAR ranges on the preclinical properties of ADCs using a maytansinoid cytotoxic agent, we prepared a series of conjugates with a cleavable linker (M9346A-sulfo-SPDB-DM4 targeting folate receptor alpha) or an uncleavable linker (J2898A-SMCC-DM1 targeting epidermal growth factor receptor) with varying DAR, and evaluated their biochemical characteristics, in vivo stability, efficacy, and tolerability.

Both M9346A-sulfo-SPDB-DM4 and J2898A-SMCC-DM1 conjugates with low (average ∼2, range 0-4) to very high DAR (average 10, range 7-14) were prepared in good yield, high monomer content and low free drug levels. At constant Ab concentration, the in vitro potency consistently increased with increasing DAR. We then characterized the in vivo disposition of these ADCs. First, pharmacokinetic analysis showed that conjugates with an average DAR below ∼6 had comparable clearance rates, but at an average DAR around 9-10 rapid clearance was observed. Biodistribution studies in mice showed that these 9-10 DAR conjugates rapidly distribute to the liver, with the maximum%ID/g for this organ at 24-28% compared with 7-10% for lower DAR conjugates (all at 2-6 h post-injection). We further studied the efficacy of these ADCs in mouse xenograft models at both constant antibody and constant maytansinoid doses. At constant Ab dose, conjugates with average DAR ∼6 are more efficacious than conjugates with DAR ∼3. However, DAR 9-10 conjugates are either less or similarly active compared to DAR 6 conjugates, consistent with the observed rapid clearance. At constant maytansinoid dose, conjugates with 2 - 6 DAR show similar efficacy, while those with 9-10 DAR were again less active. Tolerability studies monitoring body weight loss show that conjugates with different DAR, even as high as 9-10, have comparable tolerability based on total administered maytansinoid dose.

In summary, our preclinical findings on tolerability and efficacy suggest that maytansinoid conjugates with DAR in the range between 2 - 6 have better therapeutic index than conjugates with very high DAR (∼9-10). These very high DAR ADCs suffer from decreased efficacy likely due to faster clearance. These results support the use of DAR 3-4 for maytansinoid ADCs, but suggest the exploration of lower or higher DAR depending on the biology of the target antigen.

Citation Format: Xiuxia Sun, Jose F. Ponte, Nicholas C. Yoder, Jennifer Coccia, Leanne Lanieri, Rassol Laleau, Qifeng Qiu, Rui Wu, Erica Hong, Megan Bogalhas, Lintao Wang, Erin K. Maloney, Olga Ab, Hans K. Erickson, Thomas A. Keating, Ravi Chari, John M. Lambert. Effects of drug load on therapeutic index for antibody-maytansinoid conjugates. [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 4531. doi:10.1158/1538-7445.AM2015-4531

Abstract 645: Stability and efficacy comparison of site-specific and lysine-linked maytansinoid antibody-drug conjugates

As with any therapeutic molecule, antibody-drug conjugates (ADCs) exhibit structure-activity relationships, and medicinal chemistry efforts in this field strive to optimize structure to give the maximum therapeutic index. Recent interest in ADCs as cancer therapy has led to a number of different combinations of linker, payload, and conjugation chemistry.In particular, site-specific methods of payload conjugation have been suggested to generally improve therapeutic properties as compared with more established approaches directed toward lysines or endogenous cysteines.We have investigated the preparation, stability, and activity of anti-folate receptor alpha (FRα) ADCs carrying the microtubule inhibitor, DM1, and conjugated to engineered cysteine mutants utilizing different sites, and compared these ADCs with lysine-directed heterogeneous conjugates. In both embodiments, the DM1 is linked with a protease-cleavable linker. We show that highly homogeneous DM1 ADCs can be produced using engineered cysteine chemistry, enabling assessment of the effects of site-specific conjugation in cells and in animal models. We find that in vitro potency of both lysine-linked and engineered cysteine-linked ADCs against FRα-positive KB cells scales with the total DM1 delivered to cells. Buffer stability experiments in the presence of excess thiol suggest that most engineered cysteine conjugates are comparable in stability to the lysine-linked ADC. A notable exception shows about twice as much fractional DM1 loss upon 3 days of incubation as the other conjugates. Comparison of in vivo activity of two site-specific DM1 ADCs in a KB xenograft model shows measurable activity differences between different conjugation sites. However, a lysine-linked conjugate using almost identical linker chemistry shows approximately 2-fold superior activity to either site-specific construct on a molar DM1 basis. We conclude that, while site-specific conjugation of ADCs may provide a benefit in certain contexts, in other contexts, it may lead to decreased activity, such as in the anti-FRα/KB model examined here. We also observe that different conjugation sites may offer significant differences in activity. It is therefore advisable to evaluate each unique combination of payload, linker, drug:antibody ratio, conjugation site(s), and antibody to the maximum extent possible.

Citation Format: Nicholas C. Yoder, Chen Bai, Daniel Tavares, Wayne C. Widdison, Olga Ab, Kathleen R. Whiteman, Alan Wilhelm, Erin K. Maloney, Hans K. Erickson, Thomas A. Keating. Stability and efficacy comparison of site-specific and lysine-linked maytansinoid antibody-drug conjugates. [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 645. doi:10.1158/1538-7445.AM2015-645

Abstract 5442: Microscale methods for preparation and screening of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are hybrid biotherapeutics that combine the targeting specificity of monoclonal antibodies with chemically conjugated, highly potent small molecule chemotherapeutics. Using established research scale approaches, the amount of antibody material needed to prepare candidate ADCs far exceeds the quantities required for initial in vitro screening. The need to scale up production across many antibodies slows down early lead selection efforts and wastes material. We have therefore developed methods for conjugating multiple antibodies with ADC payloads in parallel at the 50-150 μg scale in 96-well plates. Pilot reactions show that antibodies can be titrated to different final drug:antibody ratios (DARs) with different payloads, and that differences in pH can alter the reaction kinetics with useful effects. We show that 96-well centrifugal ultrafiltration enables highly parallel ADC purification while maintaining the rigorous removal of residual cytotoxic impurities observed with more established methods such as gel filtration chromatography. In addition, we describe characterization of microscale ADCs using a single chromatographic assay requiring ∼5 μg of material. The resulting platform reduces the required input quantity of antibody required for in vitro ADC screening by at least 5-10 fold. It also enables much higher conjugation throughput with concomitant decrease in time needed to generate and characterize ADCs.

To assess the utility of the platform for ranking candidate antibodies, we compare in vitro cytotoxicity results for a panel of ADCs produced by both microscale and research scale methods. We also present three example screens in which antibody libraries of 10-85 members against different targets were conjugated using microscale methods and the resulting ADCs ranked by in vitro potency. For each antibody library, the screen size, conjugation conditions, and target DAR range were adjusted to suit the target biology, antibody type,and payload class. For example, we present evidence suggesting that, for targets in which functional antibody activity is not observed, normalization of DAR to the 2-6 range is adequate for screening. Across the screens, the success rates for producing ADC in quantity and quality suitable for screening were in the 75-90% range, using 200-600 μg of input antibody. Cytotoxic potencies ranging over 2-3 orders of magnitude were observed in the resulting ADC libraries, suggesting that microscale conjugation can rapidly focus ADC discoverycampaigns on high potency molecules.

For early stage antibody and ADC screening efforts, we find that microscale conjugation methods yield ADCs that can substitute for traditionally prepared conjugates. We expect these methods will be applicable across many different ADC targets and payloads, and possibly applicable more generally to conjugated macromolecule therapeutic or diagnostic reagents.

Citation Format: Nicholas C. Yoder, Kalli C. Catcott, Molly A. McShea, Carl Uli Bialucha, Parmita Saxena, Chen Bai, Kathy L. Miller, Thomas G. Gesner, Mikias Woldegiorgis, Stuart W. Hicks, Megan E. Lewis, Michael S. Fleming, Hans K. Erickson, Seth E. Ettenberg, Thomas A. Keating. Microscale methods for preparation and screening of antibody-drug conjugates. [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 5442. doi:10.1158/1538-7445.AM2015-5442

Abstract 5677: The anti-EGFR ADC, IMGN289 displays favorable pharmacokinetic properties

IMGN289 is an antibody-drug conjugate (ADC) in preclinical development for the treatment of EGFR-positive cancers. IMGN289 utilizes the same SMCC thioether linker and DM1 cytotoxic agent used in trastuzumab emtansine (T-DM1), and both conjugates contain approximately 3.5 DM1 molecules per antibody. The stability of the SMCC linker is exemplified by several reports that have shown maytansinoid release only after hydrolysis of the antibody backbone within cells and tissue to yield lysine-SMCC-DM1. Despite these findings, pharmacokinetic (PK) data for T-DM1 point to a faster clearance for conjugated antibody than for total antibody. Additionally, reports of payload release from non-maytansinoid, cysteine-linked ADCs via thiol-maleimide exchange in plasma have led to speculation of DM1 release from T-DM1, and of this occurring via a similar mechanism.

Consequently, we conducted a series of studies in mice to examine the PK behavior of IMGN289, its unmodified antibody component (J2898A) and a version of IMGN289 (J2898A-(CH2)3-DM1) in which the SMCC linker is replaced with an all-carbon linker incapable of cleavage via thiol-maleimide exchange. PK was assessed in CD-1 mice administered a single bolus iv dose of the test article. Total antibody and ADC concentrations (species with at least one linked DM1) were measured at periodic intervals over 35 days using standard sandwich ELISA assays. We also employed radioactive assays to directly measure the antibody and ADC components in plasma. For total antibody detection, the antibody or ADC was labeled with the radioactive tracer, N-succinimidyl-2,3-[3H]propionate. For ADC detection, tritium was incorporated into the C20-methoxy group of its maytansinoid moiety.

The PK profiles for the ADCs and their unmodified antibody were found to be similar using the ELISA and radioactive assays for total antibody, having only slight changes in the PK parameters for the ADC. Using the ELISA assays, we found the total antibody and ADC PK profiles were indistinguishable for IMGN289, in contrast to the reported PK for T-DM1 in mice. The profiles developed using the radioactive assays were in good agreement with these findings, supporting that the ELISA assays were reliable. To confirm our results were not unique to IMGN289, we repeated the experiments with two additional antibody-SMCC-DM1 conjugates and found similar results. Additionally, the PK profile for the all- carbon linked conjugate, J2898A-C3-[3H]DM1, was similar to that of J2898A-SMCC-[3H]DM1 conjugate, suggesting little if any thiol-maleimide exchange in the latter, consistent with the finding of similar total antibody and ADC PK profiles for Ab-SMCC-DM1 conjugates.

In summary, IMGN289 appears to retain the PK properties of its antibody component and avoids the payload release via thiol-maleimide exchange described for cysteine-maleimide linked ADCs.

Citation Format: Jose F. Ponte, Xiuxia Sun, Nicholas C. Yoder, Nathan Fishkin, Sharon Wilhelm, Susan J. Hawes, Wayne Widdison, Robert A. Mastico, Jan Pinkas, Ravi J. Chari, Robert J. Lutz, Hans K. Erickson. The anti-EGFR ADC, IMGN289 displays favorable pharmacokinetic properties. [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 5677. doi:10.1158/1538-7445.AM2013-5677

Catch-and-release probes applied to semi-intact cells reveal ubiquitin-specific protease expression in Chlamydia trachomatis infection

Protein ubiquitylation controls many cellular pathways, and timely removal of ubiquitin by deubiquitylating enzymes (DUBs) is essential to govern these different functions. To map endogenous expression of individual DUBs as well as that of any interacting proteins, we developed a catch-and-release ubiquitin probe. Ubiquitin was equipped with an activity-based warhead and a cleavable linker attached to a biotin affinity-handle through tandem site-specific modification, in which we combined intein chemistry with sortase-mediated ligation. The resulting probe is cell-impermeable and was therefore delivered to the cytosol of perfringolysin O (PFO)-permeabilized cells. This allowed us to retrieve and identify 34 DUBs and their interacting partners. We also noted the expression, in host cells infected with Chlamydia trachomatis, of two additional DUBs. Furthermore, we retrieved and identified chlamydial DUB1 (ChlaDUB1) and DUB2 (ChlaDUB2), demonstrating by experiment that ChlaDUB2, the presence and activity of which had not been detected in infected cells, is in fact expressed during the course of infection.

Use of fungal derived polysaccharide-conjugated particles to probe Dectin-1 responses in innate immunity

The number of life-threatening fungal infections has risen in immunocompromised patients, and identification of the rules that govern an appropriate immune response is essential to develop better diagnostics and targeted therapeutics. The outer cell wall component on pathogenic fungi consists of β-1,3-glucan, and Dectin-1, a pattern recognition receptor present on the cell surface of innate immune cells, binds specifically to this carbohydrate. A barrier in understanding the exact immunological response to pathogen-derived carbohydrate epitopes is the presence of multiple types of carbohydrate moieties on fungal cell walls. To dissect the immunological mechanisms used to recognize pathogens, a system of "fungal like particles" was developed that consisted of polystyrene beads, which mimicked the three dimensional shape of the fungus, coated covalently with purified β-1,3-glucan derived from Saccharomyces cerevisiae. The morphology of the β-1,3-glucan layer was examined by immunofluorescence, flow cytometery, and immuno-transmission electron microscopy. The covalent linkages of the β-1,3-glucan to the polystyrene surface were stable after subjecting the beads to detergents. By pre-treating β-1,3-glucan beads with laminarinase, a specific β-1,3-gluconase, the reactivity of the anti-β-1,3-glucan antibody was abrogated in comparison to treatment with proteinase K indicating that the coating of these beads was predominantly β-1,3-glucan. TNF-α was also measured by stimulating bone-marrow derived macrophages with the β-1,3-glucan beads, and showed a dose dependent response compared to soluble β-glucan, insoluble β-1,3-glucan, uncoated beads, and soluble β-1,3-glucan mixed with uncoated beads. Finally, β-1,3-glucan beads were incubated with GFP-Dectin-1 expressing macrophages and imaged using confocal microscopy. β-1,3-beads were taken up within minutes and retained Dectin-1 recruitment to the phagosome as compared to uncoated beads. These data describe a unique fungal-like particle system that will permit immunologists to probe the critical steps in early recognition of pathogen-derived fungal carbohydrate antigens by innate immune cells.

Site-specific N- and C-terminal labeling of a single polypeptide using sortases of different specificity

The unique reactivity of two sortase enzymes, SrtA_staph from Staphylococcus aureus and SrtA_strep from Streptococcus pyogenes, is exploited for site-specific labeling of a single polypeptide with different labels at its N and C termini. SrtA_strep is used to label the protein’s C terminus at an LPXTG site with a fluorescently labeled dialanine nucleophile. Selective N-terminal labeling of proteins containing N-terminal glycine residues is achieved using SrtA_staph and LPXT derivatives. The generality of N-terminal labeling with SrtA_staph is demonstrated by near-quantitative labeling of multiple protein substrates with excellent site specificity.

Structure and thermotropic phase behavior of fluorinated phospholipid bilayers: a combined attenuated total reflection FTIR spectroscopy and imaging ellipsometry study

Lipid bilayers consisting of lipids with terminally perfluoroalkylated chains have remarkable properties. They exhibit increased stability and phase-separated nanoscale patterns in mixtures with nonfluorinated lipids. In order to understand the bilayer properties that are responsible for this behavior, we have analyzed the structure of solid-supported bilayers composed of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) and of a DPPC analogue with 6 terminal perfluorinated methylene units (F6-DPPC). Polarized attenuated total reflection Fourier-transform infrared spectroscopy indicates that for F6-DPPC, the tilt of the lipid acyl chains to the bilayer normal is increased to 39 degrees as compared to 21 degrees for native DPPC, for both lipids in the gel phase. This substantial increase of the tilt angle is responsible for a decrease of the bilayer thickness from 5.4 nm for DPPC to 4.5 nm for F6-DPPC, as revealed by temperature-controlled imaging ellipsometry on microstructured lipid bilayers and solution atomic force microscopy. During the main phase transition from the gel to the fluid phase, both the relative bilayer thickness change and the relative area change are substantially smaller for F6-DPPC than for DPPC. In light of these structural and thermotropic data, we propose a model in which the higher acyl-chain tilt angle in F6-DPPC is the result of a conformational rearrangement to minimize unfavorable fluorocarbon-hydrocarbon interactions in the center of the bilayer due to chain staggering.

Nanoscale patterning in mixed fluorocarbon-hydrocarbon phospholipid bilayers

A growing body of literature suggests that fluorocarbons can direct self-assembly within hydrocarbon environments. We report here the fabrication and characterization of supported lipid bilayers (SLBs) composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and a synthetic, fluorocarbon-functionalized analogue, 1. AFM investigation of these model membranes reveals an intricate, composition-dependent domain structure consisting of approximately 50 nm stripes interspersed between approximately 1 microm sized domains. Although DSC of 1 showed a phase transition near room temperature, DSC of DPPC:1 mixtures exhibited complex phase behavior suggesting domain segregation. Finally, temperature-dependent AFM of DPPC:1 bilayers shows that, while the stripe structures can be melted above the Tm of 1, the stripes and domains result from immiscibility of the hydrocarbon and fluorocarbon lipid gel phases. Fluorination appears to be a promising strategy for chemical self-assembly in two dimensions. In particular, because no modification is made to the lipid headgroups, it may be useful for nanopatterning biologically relevant ligands on bilayers in vitro or in living cells.

The Role of Conserved Histidines in the Structure and Stability of Human Papillomavirus Type 16 E2 DNA-Binding Domain

The E2 protein of papillomavirus is the key regulator of viral transcription and replication. Dimerization, which takes place via its conserved C-terminal DNA-binding domain (DBD), is critical for these functions. The presence and conservation of two histidines (H290 and H320) at or near the dimer interface suggests the importance of their roles in protein structure and stability that was explored by mutating them to neutral alanine. The H290A mutant but not the H320A mutant showed a significant change in the secondary as well as tertiary structure, as monitored by far- and near-UV circular dichroism and fluorescence. We show that the wild-type DBD was more stable than either of the two histidine mutants at pH 7.5 but that the order of stability changed with pH, where, at pH 4.5, the H290A mutant was most stable. Although H290 is important for pH dependence of the stability, it is not critical for dimerization or folding. The determination of pKa values for the solvent-exposed histidine residues shows that the surface properties of the protein change with pH, suggesting different interactions that can be made by the protein in response to cellular acidification. Moreover, identification of residues crucial for E2 stability will help in the design of modified proteins with desired characteristics.

Bioorthogonal noncovalent chemistry: fluorous phases in chemical biology

Chemical entities designed to noncovalently interact with predetermined partners have fashioned a new paradigm in chemical biology. Fluorocarbons are extremely promising as supramolecular synthons toward these objectives. Bioorthogonal noncovalent interactions provide a way to modulate self-assembled systems in environments where such control has hitherto not been possible. Fluorocarbons have now found applications in self-assembly as well as proteomics, biomolecule purification and in the creation of microarray platforms. Other self-assembly motifs with similar attributes might be exploited using the same general approach.

Selective protein-protein interactions driven by a phenylalanine interface

Highly specific protein-protein interfaces have been the subject of considerable study for their potential utility in disrupting or interrogating cellular signaling and control networks. We report that coiled-coil sequences decorated with phenylalanine core residues fold into stable alpha-helical bundles and that these self-sort from similar peptide assemblies with aliphatic core side chains. For self-assembled ensembles derived from 30-residue monomeric peptides, the DeltaG of specificity is -1.5 kcal/mol, comparable with earlier self-sorting coiled-coil systems. Intriguingly, although this interface is constructed from canonical amino acids, it does not appear to have been exploited in native proteins.

Fluorinated amino acids in protein design and engineering

Selective incorporation of unnatural amino acids into proteins is a powerful tool for illuminating the principles of protein design. In particular, fluorinated amino acids have recently emerged as valuable building blocks for designing hyperstable protein folds, as well as directing highly specific protein-protein interactions. We review the collagen mimetic and coiled coil peptide systems that exemplify generalizable paradigms for future design. The unique electronic and phase properties of fluorocarbons are discussed, and protein synthesis using unnatural amino acids is briefly reviewed.