Abstract 2927: ITGB6 ProTriTAC™, a protease-activated T cell engager prodrug targeting Integrin-β6 for the treatment of solid tumors

Integrin-β6 (ITGB6) is a transmembrane protein that partners exclusively with the integrin-αV subunit to form the integrin-αVβ6 heterodimer. Due to its roles in tissue remodeling and cancer pathogenesis, therapeutic targeting of integrin-αVβ6 has been investigated for both fibrotic diseases and cancers. ITGB6 is homogenously overexpressed in numerous solid tumor types, including breast, head and neck, pancreatic, and non-small cell lung cancers. Additionally, its expression is an unfavorable prognostic marker in multiple cancers. However, past attempts to therapeutically target ITGB6 have been hindered by its expression in several normal tissues, including bladder, skin, lung, kidney, and muscle. For example, clinical testing of an integrin αVβ6-targeting monoclonal antibody was discontinued reportedly due to toxicity concerns. To reduce on-target/off-tumor toxicities from ITGB6 expression in normal tissues and to enable safe targeting of ITGB6 in solid tumors, we developed an ITGB6-targeting ProTriTAC™ (Protease-activated Tri-specific T cell Activating Construct), a T cell engager prodrug which is engineered to be preferentially activated in tumor tissue. ProTriTACs consists of three humanized antibody-derived binding domains on a single polypeptide chain: anti-albumin for half-life extension, anti-CD3 for T cell engagement, and anti-target antigen for tumor cell engagement. The anti-albumin domain, bearing a masking moiety and a protease-cleavable linker, keeps the prodrug inert by inhibiting binding of the adjacent anti-CD3 domain to T cells. Cleavage of the linker by tumor-associated proteases removes the anti-albumin domain along with the masking moiety to reveal a potent active drug that directs T cell killing within the tumor. As designed, in vitro binding and functional assays show that the protease-activated T cell engager has >1000-fold improved binding to human T cells and ~100-fold improved T cell killing compared to the intact masked prodrug. In immunodeficient mice reconstituted with human T cells, anti-ITGB6 ProTriTAC demonstrated robust anti-tumor activity in multiple ITGB6-expressing xenograft tumor models, including HCC70 (breast), CAL27 (head and neck), and HPAFII (pancreatic), with complete tumor regression as low as 30 μg/kg. To determine the potential toxicity and toxicokinetic characteristics of anti-ITGB6 ProTriTAC, an intra-animal dose escalation was conducted in cynomolgus monkeys. Starting at 20 μg/kg with weekly 3-fold dose escalations, anti-ITGB6 ProTriTAC was well tolerated up to 540 μg/kg, which was the highest dose tested. The preclinical activity in rodent tumor models, coupled to its tolerability in cynomolgus monkeys, support the development of anti-ITGB6 ProTriTAC as a therapeutic in a broad range of ITGB6-expressing solid tumors.

Citation Format: Regina J. Lin, Sony S. Rocha, Maria R. Dayao, Subramanian Thothathri, Raphaela Rose Banzon, Kevin J. Wright, Wade Aaron, Yinghua Xiao, Nick Bergo, Linh To, Mabel Bush, Manasi Barath, Yi Yang, Timothy Yu, Willis Kwong, Hubert Situ, Eric Bragg, Jessica O'Rear, Kevin Carlin, Stephen Yu, Maritza Solorio, Bryan Lemon, Richard Austin, Holger Wesche, S. Jack Lin. ITGB6 ProTriTAC™, a protease-activated T cell engager prodrug targeting Integrin-β6 for the treatment of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2927.

Abstract 2928: TROP2 ProTriTAC™, a protease-activated T cell engager prodrug targeting TROP2 for the treatment of solid tumors

Tumor-associated calcium signal transducer 2 (Trop2) is a cell surface glycoprotein that promotes cell renewal, proliferation and tumorigenesis. Trop2 is homogenously overexpressed in numerous solid tumor types, including breast, head and neck, prostate, bladder, and non-small cell carcinoma; however, Trop2 is also expressed in several normal tissues, including skin, respiratory tract, and female reproductive organs. Although Trop2-targeted antibody drug conjugates (ADCs) have demonstrated clinical activity, with regulatory approvals in subsets of breast and bladder cancers, their broader utility is still hampered by off-target payload-mediated toxicities. To address this issue, and to avoid on-target off-tumor toxicities, we developed a Trop2-targeting ProTriTAC™ (Protease-activated Tri-specific T cell Activating Construct), a T cell engager prodrug which is engineered to be preferentially activated in tumor tissue. ProTriTACs consists of three humanized antibody-derived binding domains on a single polypeptide chain: anti-albumin for half-life extension, anti-CD3 for T cell engagement, and anti-target antigen for tumor cell engagement. The anti-albumin domain, bearing a masking moiety and a protease-cleavable linker, keeps the prodrug inert by inhibiting binding of the adjacent anti-CD3 domain to T cells. Cleavage of the linker by tumor-associated proteases removes the anti-albumin domain along with the masking moiety to reveal a potent active drug that directs T cell killing within the tumor. As designed, in vitro binding and functional assays show that the protease-activated T cell engager has >1000-fold improved binding to primary human T cells and ~100-fold improved T cell killing compared to its intact masked prodrug. In immunodeficient mice reconstituted with human T cells, anti-Trop2 ProTriTAC demonstrated robust anti-tumor activity in multiple xenograft tumor models, including HCC70 (breast), CAL27 (head and neck), and HPAFII (pancreatic), with complete tumor regression as low as 30 μg/kg. To determine the potential toxicity and toxicokinetic characteristics of anti-Trop2 ProTriTAC, an intra-animal dose escalation was conducted in cynomolgus monkeys until individual maximum tolerated dose (MTD). The starting dose was 20 μg/kg with weekly 3-fold dose escalations until MTD. The one-month study revealed that anti-Trop2 ProTriTAC was tolerated at the 180 μg/kg dose level, but not at the next higher dose level of 540 μg/kg. Toxicokinetics, gross pathology, histopathology, and cytokine data will be presented. The preclinical activity in rodent tumor models, coupled to its tolerability in cynomolgus monkeys, support the development of anti-Trop2 ProTriTAC as a therapeutic in a broad range of Trop2-expressing solid tumors.

Citation Format: Sony S. Rocha, Regina Lin, Maria R. Dayao, Raphaela Rose Banzon, Subramanian Thothathri, Kevin J. Wright, Wade Aaron, Yinghua Xiao, Nick Bergo, Linh To, Mabel Bush, Manasi Barath, Timothy Yu, Willis Kwong, Hubert Situ, Eric Bragg, Jessica O'Rear, Kevin Carlin, Stephen Yu, Maritza Solorio, Bryan Lemon, Richard Austin, Holger Wesche, S. Jack Lin. TROP2 ProTriTAC™, a protease-activated T cell engager prodrug targeting TROP2 for the treatment of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2928.

Abstract 2861: TriTAC-XR: An extended-release T cell engager platform designed to minimize cytokine release syndrome

CD3-targeted T cell engagers are potent anti-tumor therapies, but their development often requires management of cytokine release syndrome (CRS). One strategy to reduce CRS is subcutaneous dosing, which is hypothesized to mitigate CRS by reducing the maximum drug concentration (Cmax) and preserve efficacy by maintaining the same minimum drug concentration (Cmin) as intravenous dosing. Although promising for mitigating CRS, this approach is limited by its increased immunogenicity risks. A T cell engager designed to be dosed intravenously but engineered to mimic the PK properties of subcutaneous dosing could alleviate CRS without increasing immunogenicity. Here we describe TriTAC-XR, a platform of T cell engager prodrugs designed to become slowly activated in systemic circulation. This extended-release mechanism results in a slow build-up of circulating active drug and minimizes the Cmax/Cmin ratio, similar to subcutaneous dosing. TriTAC-XR prodrugs were engineered by adding a peptide mask and protease-cleavable linker to the N-terminus of a TriTAC, a constitutively active and half-life extended T cell engager. The mask binds to the anti-CD3ε domain and inhibits T cell binding. Upon cleavage by systemic proteases, active T cell engager is released. In vitro, TriTAC-XR has markedly reduced binding to recombinant CD3ε protein and to primary T cells as well as reduced potency in functional T cell-dependent cellular cytotoxicity (TDCC) assays compared to its unmasked active drug. In cynomolgus monkeys, TriTAC-XRs targeting multiple tumor antigens resulted in a gradual build-up of active drug during the first week post dose and significantly lower Cmax/Cmin ratios than comparable constitutively active TriTACs. Modeling based on these PK data predicts that TriTAC-XR dosed intravenously will result in a slower build-up of active drug and smaller Cmax/Cmin ratios than TriTAC dosed intravenously or subcutaneously. Cytokine release and target cell depletion in cynomolgus monkeys were used to compare the therapeutic index of TriTAC-XR to TriTAC. A single dose of FLT3-targeting TriTAC-XR resulted in 100-fold protection in cytokine release and similar FLT3 expressing cell depletion when compared to an equivalent FLT3-targeting TriTAC. Similarly, repeat dosing of a TriTAC-XR targeting B cells resulted in complete B cell depletion with substantially lower cytokines than a comparable TriTAC. TriTAC-XR is expected to improve the safety of T cell engagers by reducing CRS and may increase clinical dosing convenience by enabling higher doses that will extend dosing intervals.

Citation Format: Kathryn Strobel Kwant, Sony S. Rocha, Timothy Yu, Katrina Stephenson, Raphaela Rose Banzon, Sydney Vollhardt, Golzar Hemmati, Evan Callihan, Wade H. Aaron, Subramanian Thothathri, Jessica O'Rear, Eric Bragg, Willis Kwong, Hubert Situ, Avneel Hundal, Stephen Yu, Taggra Jackson, Kevin Carlin, Yinghua Xiao, Maria Rosalyn Dayao, Linh To, Nick Bergo, Kevin Wright, Richard Austin, Holger Wesche, Bryan Lemon, S. Jack Lin. TriTAC-XR: An extended-release T cell engager platform designed to minimize cytokine release syndrome [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 2861.

867 TriTAC-XR is an extended-release T cell engager platform designed to minimize cytokine release syndrome by reducing Cmax in systemic circulation

Background

CD3-targeted T cell engagers are potent anti-tumor therapies, but their development often requires management of cytokine release syndrome (CRS). Subcutaneous dosing is a promising strategy to reduce CRS, but its application is limited by its increased immunogenicity risks. Subcutaneous dosing is hypothesized to mitigate CRS by reducing the maximum drug concentration (Cmax) and preserve efficacy by maintaining the same minimum drug concentration (Cmin) as intravenous dosing. A T cell engager designed to be dosed intravenously but engineered to mimic the PK properties of subcutaneous dosing could alleviate CRS without increasing immunogenicity.

Methods

TriTAC-XR molecules are engineered T cell engager prodrugs that become slowly activated in systemic circulation. This extended-release mechanism results in a slow build-up of circulating active drug, similar to subcutaneous dosing, and extends drug exposure to enable longer dosing intervals. The prodrug was engineered by adding a peptide mask and protease-cleavable linker to the N-terminus of a TriTAC, a constitutively active and half-life extended T cell engager. The mask binds to the anti-CD3ε domain and prevents T cell binding. Upon cleavage by systemic proteases, active T cell engager is released. Binding was assessed using ELISA on recombinant CD3ε protein and using flow cytometry on primary T cells. T cell engager function was assessed using T cell-dependent cellular cytotoxicity (TDCC) assays with resting human T cells. Safety and efficacy were modeled in non-human primates.

Results

TriTAC-XR had markedly reduced binding to recombinant CD3ε protein and to primary T cells as well as reduced potency in functional TDCC assays compared to its unmasked active drug. In cynomolgus monkeys, TriTAC-XR had significantly attenuated cytokine production while maintaining comparable pharmacodynamic effects as a non-masked active drug. The ratio of Cmax to Cmin for the active TriTAC-XR was significantly smaller than a non-masked control.

Conclusions

TriTAC-XR is activated in a time released manner by systemic proteases to minimize differences between the Cmax and Cmin of systemic active drug. This mechanism is different from other protease-activated T cell engager prodrugs that are only activated by tumor-associated proteases. Compared to canonical T cell engagers, TriTAC-XR is expected to improve safety by reducing CRS and to provide convenience by extending dosing intervals.

First-in-human phase I study of HPN424, a tri-specific half-life extended PSMA-targeting T-cell engager in patients with metastatic castration-resistant prostate cancer (mCRPC)

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Background: HPN424 is a first-in-class, prostate-specific membrane antigen (PSMA)-targeting T-cell engager designed as a small, globular protein to enable efficient solid-tumor penetration with prolonged half-life. HPN424 is derived from the TriTAC platform (Tri-specific T-Cell-Activating Construct) and engineered with three binding domains: anti-PSMA for tumor cell engagement, anti-albumin for half-life extension and anti-CD3 for T-cell engagement. Methods: This Ph I study is evaluating HPN424 in progressing mCRPC patients (pts) who have received >2 prior systemic therapies. Primary endpoints are safety, tolerability and determination of MTD/RP2D. Secondary objectives are pharmacokinetics (PK), pharmacodynamics, immunogenicity, and preliminary anti-tumor activity. HPN424 is administered IV once weekly. Tumor assessments include PSA, CT, and bone scans every 9 weeks. Results: As of 1/17/20, 27 pts were dosed in 8 cohorts ranging from 1.3 to 72ng/kg. Pts received a median of 6 prior systemic regimens, including >1 novel AR therapy, and 59% received prior chemotherapy for mCRPC. Median PSA at baseline was 251 ng/mL (range: 0.05 – 5000). No DLTs have been observed. The most common grade >3 TRAEs were cytokine release syndrome (CRS) (3 pts) and transient elevated liver transaminases (2 pts) that occurred concurrently with CRS. All CRS events resolved and pts were successfully re-treated. Short-term steroid premedication was effective in limiting CRS and allowing long-term weekly treatment. HPN424 demonstrated dose proportional increase in Cmax and AUC with a geometric mean T1/2 of 30.5 hours. Dose-dependent, transient increases in peripheral cytokine and chemokine levels were observed. Reduction in circulating tumor cells (CTCs) was seen in 11 of 19 pts with measurable CTC at baseline. Six pts had PSA decreases from baseline ranging from -3.8% to -76%, including 2 pts with PSA decline ≥50%. Ten of 20 pts (50%) with > 18 weeks follow-up remained on study beyond week 18 and includes 8 pts on study > 24 weeks. Conclusions: HPN424 represents a novel half-life extended PSMA-targeting T-cell engager that can be safely administered once weekly. AEs have been transient and manageable. Cytokine increases indicate T-cell activation. CTC reductions in subset of pts suggest target engagement. Early signs of clinical activity include PSA reductions and time on study, including 8 pts on study > 24 weeks. Dose escalation is ongoing, including exploration of step dosing. Clinical trial information: NCT03577028 .

Abstract C033: HPN328: An anti-DLL3 T cell engager for treatment of small cell lung cancer

HPN328: An Anti-DLL3 T Cell Engager for Treatment of Small Cell Lung Cancer Delta-like protein 3 (DLL3) is a Notch inhibitory ligand is expressed in more than 70% of small cell lung cancers (SCLCs), while there is little to no surface expression in normal adult tissues outside of the CNS *1. SCLC is an aggressive neuroendocrine tumor that represents about 15 percent of all lung cancers. Although often responsive to standard of care treatment, relapse is common, with a median progression-free survival of 2–3 months and median overall survival (OS) of 8–13 months and 5-year OS rate of <5%*2. DLL3 thus represents an attractive potential target for T cell–redirecting immunotherapy in SCLC. HPN328 is a tri-specific T cell activating construct (TriTAC) consisting of three binding domains: a N-terminal single chain Fv (scFv) that binds to CD3ϵ of the T cell receptor (TCR), a middle single domain antibody (sdAb) that binds to human serum albumin (HSA) to extend the half-life, and a C-terminal sdAb that binds to human DLL3. HPN328 is designed to simultaneously engage DLL3 on a target SCLC cell and CD3 on a T cell resulting in T cell activation, proliferation, and the eventual lysis of the target lung cancer cell. HPN328 is a highly stable single polypeptide of ~ 53 kDa expressed in CHO cells. It binds to human and cynomolgus monkey DLL3, albumin, and CD3ϵ with similar affinities. Flow cytometry analysis of T cells from various normal donors and a panel of DLL3 positive and DLL3 negative tumor cell lines confirmed binding of HPN328 to its native targets expressed on the cell surface. HPN328 induces potent killing of DLL3 expressing SCLC cell lines in vitro. In co-cultures of T cells from normal human donors, target tumor cells, and HSA, HPN328 mediated dose-dependent and DLL3-dependent cytotoxicity. Concomitant with target tumor cell killing, HPN328 also mediated dose-dependent upregulation of CD25 and CD69 on T cells in the TDCC co-cultures when DLL3 positive tumor cells were present. Consistent with the mechanism of action of CD3-based T cell engaging molecules, T cell derived cytokines, including TNFα, IL-2 and IFNγ, were detected. Nonclinical in vivo properties of HPN328 were evaluated in an NCI-H82 SCLC established tumor model. When administered to mice bearing human SCLC xenografts and human T cells, HPN328 eradicated subcutaneous tumors. In a single dose pilot toxicity study in cynomolgus monkeys, HPN328 was well tolerated at 1 and 10 mg/kg. Pharmacodynamic changes such as transient cytokine elevation mainly at the high dose were observed, consistent with the expected mechanism of action of T cell engagers. There were no clinically significant or adverse test article-related changes in hematology or clinical chemistry, and no apparent adverse findings at terminal and recovery necropsy. HPN328 exhibited linear PK properties in the given dose range with a serum half-life of 64 to 85 hours. Pharmacokinetic analysis supports weekly administration of HPN328 in humans. Preclinical and nonclinical characterization suggests that HPN328 is a highly efficacious, safe, and novel therapeutic candidate. A first-in-human phase 1 clinical trial is planned to evaluate HPN328 in SCLC. *1. Saunders, L et al. (2015) Sci Transl Med. 7(302): 302ra136. *2. Navarro, A et al. (2017) Transl Lung Cancer Res. 6(1): S78–S83.

Citation Format: Wade H Aaron, Richard Austin, Manasi Barath, Evan Callihan, Michael Cremin, Thomas Evans, Maria Gamez, Vaishnavi Ganti, Golzar Hemmati, Kathryn Kwant, Che-Leung Law, Bryan Lemon, Llewelyn Lao, Mary Ellen Molloy, Jessica O’Rear, Laura Sun, Holger Wesche, Stephen Yu, Timothy Yu. HPN328: An anti-DLL3 T cell engager for treatment of small cell lung cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C033. doi:10.1158/1535-7163.TARG-19-C033

Abstract 1773: HPN424, a half-life extended, PSMA/CD3-specific TriTAC for the treatment of metastatic prostate cancer

Metastatic, castrate-resistant prostate cancer (mCRPC) is diagnosed in up to 50,000 patients each year in the US alone, and roughly 27,000 patients will succumb to it every year. Once metastasized beyond regional lymph nodes, the 5-year survival rate is 30%. While novel therapeutics like abiraterone and enzalutamide have improved the treatment options for mCRPC, no curative treatment is available, and new therapies are urgently needed. HPN424 is a ~50-kDa antibody derivative called TriTAC (Tri-specific T cell Activating Construct) under development for the treatment of mCRPC. It is designed to simultaneously bind to CD3ε on T cells and to prostate specific membrane antigen (PSMA, FOLH1) on prostate cancer cells. A third domain of HPN424 binds non-covalently to serum albumin for extension of serum half-life. PSMA is expressed in >90% of malignant lesions of patients, and outside the central nervous system, its expression on normal tissue is largely restricted to the prostate. HPN424 binds human PSMA with sub-nanomolar affinity. When incubated in co-cultures with resting, human T cells and prostate cancer cells, it activates T cells and induces cytokine production, proliferation and redirected target cell killing with EC50 values in the single digit picomolar range. When administered to mice bearing human prostate cancer xenografts and human T cells, HPN424 eradicates subcutaneous tumors. The affinities of HPN424 for human and cynomolgus monkey CD3 and albumin are comparable, while HPN424 binds only marginally to cynomolgus PSMA. HPN424 is very well tolerated in non-human primates, even at high doses, indicating that CD3-binding has little if any pharmacological effect in the absence of target binding. Pharmacokinetic analysis supports weekly administration in humans. Our preclinical data suggest that HPN424 will be highly efficacious, safe and convenient for the treatment of patients with mCRPC.

Citation Format: Bryan Lemon, Wade Aaron, Richard Austin, Patrick Baeuerle, Manasi Barath, Adrie Jones, Susan D. Jones, Kathryn Kwant, Che-Leung Law, Anna Muchnik, Kenneth Sexton, Laurie Tatalick, Holger Wesche, Timothy Yu. HPN424, a half-life extended, PSMA/CD3-specific TriTAC for the treatment of metastatic prostate cancer [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 1773.

Abstract 3814: TriTACs are novel T cell-engaging therapeutic proteins optimized for the treatment of solid tumors and for long serum half-life

T cell engagers are antibody-based therapeutics that transiently tether T cells via the T cell receptor complex (TCR) to surface antigens on tumor cells. This leads to activation of T cells and redirected lysis of the attached target cell. The therapeutic potential of this modality was demonstrated by blinatumomab, a CD19/CD3-bispecific T cell engager approved for the treatment of adult patients with relapsed/refractory acute lymphoblastic leukemia. Despite success of this T cell-engaging therapy in a hematologic malignancy, clinical studies in solid tumors with other T cell engagers have been less encouraging so far. The TriTAC (Tri-specific T cell Activating Construct) platform was developed to address shortcomings of existing T cell engagers, including short serum half-life, limited tissue penetration, and suboptimal activity. TriTAC constructs are made of a single polypeptide designed to bind to a cancer surface antigen, the CD3 epsilon subunit of the TCR, and to human serum albumin. CD3 is bound by a single-chain variable fragment (scFv) while both tumor targeting and albumin binding are achieved by single domain antibodies. The latter allow TriTACs to be very small, stable, and easily produced and purified. Noncovalent binding to serum albumin has been validated as an effective way to extend the serum half-life of other proteins up to several weeks. Even though TriTACs have three binding domains, their overall size is only ~50 kDa, one third of the size of a monoclonal antibody. This is expected to allow for faster diffusion into human tumor tissues than is possible with antibodies given the high interstitial pressure and dense extracellular matrix in solid tumors. TriTACs can induce T cell to kill tumor cells in vitro at single-digit picomolar to femtomolar concentrations with concomitant induction of inflammatory cytokine release and T cell proliferation. TriTACs can diffuse much faster across an extracellular matrix than antibodies, and eradicate tumors in mouse xenograft models supplemented with human T cells. In nonhuman primates, TriTAC molecules have serum half-lives of approximately 4 days, and appear well tolerated.

Citation Format: Holger Wesche, Wade Aaron, Richard J. Austin, Patrick A. Baeuerle, Adrie Jones, Bryan Lemon, Kenneth Sexton, Timothy Yu. TriTACs are novel T cell-engaging therapeutic proteins optimized for the treatment of solid tumors and for long serum half-life [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 3814.

Abstract 1781: HPN536, a T cell-engaging, mesothelin/CD3-specific TriTAC for the treatment of solid tumors

Mesothelin (MSLN) is a GPI-linked tumor antigen overexpressed in a variety of solid tumors, including ovarian, pancreatic, lung and triple-negative breast cancer. Normal tissue expression is restricted to single-cell, mesothelial layers lining the pleural, pericardial, and peritoneal cavities. Overexpression of MSLN is associated with poor prognosis in lung adenocarcinoma and triple-negative breast cancer. MSLN has been used as cancer target antigen for numerous modalities, including immunotoxins, vaccines, antibody drug conjugates and CAR-T cells. Early signs of clinical efficacy have validated MSLN as target, but therapies with improved efficacy are still needed to address the significant, unmet medical need posed by MSLN-expressing cancers. HPN536 is a ~50-kDa antibody derivative called TriTAC (Tri-specific T cell Activating Construct) designed to simultaneously bind to MSLN on tumor cells and to CD3ε on T cells with an affinity of 1 nM and 14 nM, respectively. Transient bispecific binding leads to the formation of an immunological cytolytic synapse, T cell activation and redirected tumor cell killing. A third domain of HPN536 binds non-covalently to serum albumin with an affinity of 8 nM to extend serum half-life life. Because TriTACs are built using single domain antibodies, TriTACs are much smaller than full size antibodies and are anticipated to demonstrate improved penetration of human tumors compared to full sized antibodies. HPN536 is produced by eukaryotic cell culture and secreted as a highly stable, single polypeptide. It binds with similar affinity to human and cynomolgus MSLN, albumin and CD3. When incubated in co-cultures with resting, human or cynomolgus T cells and human tumor cells, T cells are induced to release cytokines, to proliferate, and to specifically lyse MSLN-positive target cells with EC50 values at single-digit picomolar concentrations. In an exploratory toxicological study in non-human primates, HPN536 was well tolerated and showed pharmacokinetics in support of weekly dosing in humans. Preclinical characterization suggests that HPN536 is an efficacious and safe novel therapeutic candidate for the convenient treatment of patients with MSLN-expressing malignancies.

Citation Format: Richard Austin, Wade Aaron, Patrick Baeuerle, Manasi Barath, Adrie Jones, Susan D. Jones, Che-Leung Law, Kathryn Kwant, Bryan Lemon, Anna Muchnik, Kenneth Sexton, Laurie Tatalick, Holger Wesche, Timothy Yu. HPN536, a T cell-engaging, mesothelin/CD3-specific TriTAC for the treatment of solid tumors [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 1781.

Soluble CLEC2 Extracellular Domain Improves Glucose and Lipid Homeostasis by Regulating Liver Kupffer Cell Polarization

The polarization of tissue resident macrophages toward the alternatively activated, anti-inflammatory M2 phenotype is believed to positively impact obesity and insulin resistance. Here we show that the soluble form of the extracellular domain (ECD) of C-type lectin-like receptor 2, CLEC2, regulates Kupffer cell polarization in the liver and improves glucose and lipid parameters in diabetic animal models. Over-expression of Fc-CLEC2(ECD) in mice via in vivo gene delivery, or injection of recombinant Fc-CLEC2(ECD) protein, results in a reduction of blood glucose and liver triglyceride levels and improves glucose tolerance. Furthermore, Fc-CLEC2(ECD) treatment improves cytokine profiles and increases both the M2 macrophage population and the genes involved in the oxidation of lipid metabolism in the liver. These data reveal a previously unidentified role for CLEC2 as a regulator of macrophage polarity, and establish CLEC2 as a promising therapeutic target for treatment of diabetes and liver disease.

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CLEC2, a type II C-type lectin-like receptor, is expressed on a variety of cell types including Kupffer cells.

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Overexpression of CLEC2 ECD in mice improves glucose and lipid parameters and induces markers of alternatively activated Kupffer cells.

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CLEC2 is a promising therapeutic target for the treatment of diabetes and liver diseases.

Dual actions of fibroblast growth factor 19 on lipid metabolism

Elevated triglyceride (TG) and cholesterol levels are risk factors for cardiovascular disease and are often associated with diabetes and metabolic syndrome. Recent reports suggest that fibroblast growth factor (FGF)19 and FGF21 can dramatically improve metabolic dysfunction, including hyperglycemia, hypertriglyceridemia, and hypercholesterolemia. Due to their similar receptor specificities and co-receptor requirements, FGF19 and FGF21 share many common properties and have been thought to be interchangeable in metabolic regulation. Here we directly compared how pharmacological administration of recombinant FGF19 or FGF21 proteins affect metabolism in B6.V-Lep(ob)/J leptin-deficient mice. FGF19 and FGF21 equally improved glucose parameters; however, we observed increased serum TG and cholesterol levels after treatment with FGF19 but not with FGF21. Increases in serum TGs were also observed after a 4-day treatment with FGF19 in C57BL6/J mice on a high-fat diet. This is in contrast to many literature reports that showed significant improvements in hyperlipidemia after chronic treatment with FGF19 or FGF21 in high-fat diet models. We propose that FGF19 has lipid-raising and lipid-lowering actions mediated through different FGF receptors and target tissues, and the results described here provide a potential mechanism that may explain the inconsistency in the reported effects of FGF19 on lipid metabolism.

Characterization of a FGF19 variant with altered receptor specificity revealed a central role for FGFR1c in the regulation of glucose metabolism

Diabetes and associated metabolic conditions have reached pandemic proportions worldwide, and there is a clear unmet medical need for new therapies that are both effective and safe. FGF19 and FGF21 are distinctive members of the FGF family that function as endocrine hormones. Both have potent effects on normalizing glucose, lipid, and energy homeostasis, and therefore, represent attractive potential next generation therapies for combating the growing epidemics of type 2 diabetes and obesity. The mechanism responsible for these impressive metabolic effects remains unknown. While both FGF19 and FGF21 can activate FGFRs 1c, 2c, and 3c in the presence of co-receptor βKlotho in vitro, which receptor is responsible for the metabolic activities observed in vivo remains unknown. Here we have generated a variant of FGF19, FGF19-7, that has altered receptor specificity with a strong bias toward FGFR1c. We show that FGF19-7 is equally efficacious as wild type FGF19 in regulating glucose, lipid, and energy metabolism in both diet-induced obesity and leptin-deficient mouse models. These results are the first direct demonstration of the central role of the βKlotho/FGFR1c receptor complex in glucose and lipid regulation, and also strongly suggest that activation of this receptor complex alone might be sufficient to achieve all the metabolic functions of endocrine FGF molecules.

Crystal structure of a single-chain trimer of human adiponectin globular domain

Adiponectin is increasingly recognized as a potential therapeutic agent for the treatment of diabetes and other metabolic diseases. It circulates in plasma as homotrimers and higher-order oliogomers of homotrimers. To facilitate the production of active recombinant adiponectin as a therapeutic tool, we designed a single-chain globular domain adiponectin (sc-gAd) in which three monomer sequences are linked together in tandem to form one contiguous polypeptide. Here, we present the crystal structure of human sc-gAd at 2.0Å resolution. The structure reveals a similar trimeric topology to that of mouse gAd protein. Trimer formation is further rigidified by three calcium ions.

The FGFR D3 domain determines receptor selectivity for fibroblast growth factor 21

FGF21 is a member of a unique subfamily of fibroblast growth factors that function as endocrine hormones and regulate a variety of metabolic activities. Unlike paracrine FGFs, FGF21 does not bind heparin and requires βKlotho as a co-receptor to activate FGFR signaling. In the presence of βKlotho, FGF21 is able to activate FGFRs 1c, 2c and 3c but not FGFR4. Chimeric FGFR1c/FGFR4 receptors were constructed to identify domains that confer this specificity and to understand regions important for FGF21-induced receptor activation. With these chimeras, we showed that domain 3 of the FGFR1c extracellular domain plays a critical role in specificity determination and receptor activation by FGF21. Furthermore, we were able to narrow down the sequences important for this function to a six amino acid region within domain 3 of FGFR1c. It is interesting to note that this region falls into the βC'-βE loop, which has been shown to be important for receptor specificity determination in paracrine FGFs, suggesting a common principle in both endocrine and paracrine FGF receptor interaction and activation.

Separating mitogenic and metabolic activities of fibroblast growth factor 19 (FGF19)

FGF19 and FGF21 are distinctive members of the FGF family that function as endocrine hormones. Their potent effects on normalizing glucose, lipid, and energy homeostasis in disease models have made them an interesting focus of research for combating the growing epidemics of diabetes and obesity. Despite overlapping functions, FGF19 and FGF21 have many discrete effects, the most important being that FGF19 has both metabolic and proliferative effects, whereas FGF21 has only metabolic effects. Here we identify the structural determinants dictating differential receptor interactions that explain and distinguish these two physiological functions. We also have generated FGF19 variants that have lost the ability to induce hepatocyte proliferation but that still are effective in lowering plasma glucose levels and improving insulin sensitivity in mice. Our results add valuable insight into the structure-function relationship of FGF19/FGF21 and identify the structural basis underpinning the distinct proliferative feature of FGF19 compared with FGF21. In addition, these studies provide a road map for engineering FGF19 as a potential therapeutic candidate for treating diabetes and obesity.

Generation of novel long-acting globular adiponectin molecules

Adiponectin is an adipocyte-derived hormone that has been shown to play important roles in the regulation of glucose and energy homeostasis. It exists as homotrimers or complexes containing multiple homotrimer units in plasma. The recombinant adiponectin proteins have been difficult to produce, making it challenging for both research as well as potential therapeutic development. Here, we show a novel approach for the generation of globular adiponectin that involves linking three monomer sequences together in tandem to generate one continuous polypeptide, which we have termed single-chain globular adiponectin (sc-gAd). To improve the pharmacokinetic properties of sc-gAd further, we fused it to an Fc fragment. The combined effects of single-chain and Fc fusion improved the plasma half-life from less than 2 h to close to 2 weeks. Using adeno-associated virus as a delivery method, we demonstrate that Fc-sc-gAd improved both fasting glucose levels and the tolerance to a glucose challenge in ob/ob mice without changes in body weight. Therefore, our results demonstrated the feasibility of generating globular adiponectin trimers from a single polypeptide and a long-acting globular adiponectin that could serve as a starting point for adiponectin-based therapeutics. This novel approach could also be applied to other complement factor C1q family members; in particular, this opens the possibility to study the biological functions of precisely defined heterotrimers of various family members that had not been previously possible.

FGF19-induced hepatocyte proliferation is mediated through FGFR4 activation

FGF19 and FGF21, unique members of the fibroblast growth factor (FGF) family, are hormones that regulate glucose, lipid, and energy homeostasis. Increased hepatocyte proliferation and liver tumor formation have also been observed in FGF19 transgenic mice. Here, we report that, in contrast to FGF19, FGF21 does not induce hepatocyte proliferation in vivo. To identify the mechanism for FGF19-induced hepatocyte proliferation, we explored similarities and differences in receptor specificity between FGF19 and FGF21. We find that although both are able to activate FGF receptors (FGFRs) 1c, 2c, and 3c, only FGF19 activates FGFR4, the predominant receptor in the liver. Using a C-terminal truncation mutant of FGF19 and a series of FGF19/FGF21 chimeric molecules, we determined that amino acids residues 38-42 of FGF19 are sufficient to confer both FGFR4 activation and increased hepatocyte proliferation in vivo to FGF21. These data suggest that activation of FGFR4 is the mechanism whereby FGF19 can increase hepatocyte proliferation and induce hepatocellular carcinoma formation.

C-terminal tail of FGF19 determines its specificity toward Klotho co-receptors

FGF19 subfamily proteins (FGF19, FGF21, and FGF23) are unique members of fibroblast growth factors (FGFs) that regulate energy, bile acid, glucose, lipid, phosphate, and vitamin D homeostasis in an endocrine fashion. Their activities require the presence of alpha or betaKlotho, two related single-pass transmembrane proteins, as co-receptors in relevant target tissues. We previously showed that FGF19 can bind to both alpha and betaKlotho, whereas FGF21 and FGF23 can bind only to either betaKlotho or alphaKlotho, respectively in vitro. To determine the mechanism regulating the binding and specificity among FGF19 subfamily members to Klotho family proteins, chimeric proteins between FGF19 subfamily members or chimeric proteins between Klotho family members were constructed to probe the interaction between those two families. Our results showed that a chimera of FGF19 with the FGF21 C-terminal tail interacts only with betaKlotho and a chimera with the FGF23 C-terminal tail interacts only with alphaKlotho. FGF signaling assays also reflected the change of specificity we observed for the chimeras. These results identified the C-terminal tail of FGF19 as a region necessary for its recognition of Klotho family proteins. In addition, chimeras between alpha and betaKlotho were also generated to probe the regions in Klotho proteins that are important for signaling by this FGF subfamily. Both FGF23 and FGF21 require intact alpha or betaKlotho for signaling, respectively, whereas FGF19 can signal through a Klotho chimera consisting of the N terminus of alphaKlotho and the C terminus of betaKlotho. Our results provide the first glimpse of the regions that regulate the binding specificity between this unique family of FGFs and their co-receptors.

Co-receptor Requirements for Fibroblast Growth Factor-19 Signaling

FGF19 is a unique member of the fibroblast growth factor (FGF) family of secreted proteins that regulates bile acid homeostasis and metabolic state in an endocrine fashion. Here we investigate the cell surface receptors required for signaling by FGF19. We show that betaKlotho, a single-pass transmembrane protein highly expressed in liver and fat, induced ERK1/2 phosphorylation in response to FGF19 treatment and significantly increased the interactions between FGF19 and FGFR4. Interestingly, our results show that alphaKlotho, another Klotho family protein related to betaKlotho, also induced ERK1/2 phosphorylation in response to FGF19 treatment and increased FGF19-FGFR4 interactions in vitro, similar to the effects of betaKlotho. In addition, heparin further enhanced the effects of both alphaKlotho and betaKlotho in FGF19 signaling and interaction experiments. These results suggest that a functional FGF19 receptor may consist of FGF receptor (FGFR) and heparan sulfate complexed with either alphaKlotho or betaKlotho.

Structural studies of the human PBAF chromatin-remodeling complex

ATP-dependent chromatin remodeling is one of the central processes responsible for imparting fluidity to chromatin and thus regulating DNA transactions. Although knowledge on this process is accumulating rapidly, the basic mechanism (or mechanisms) by which the remodeling complexes alter the structure of a nucleosome is not yet understood. Structural information on these macromolecular machines should aid in interpreting the biochemical and genetic data; to this end, we have determined the structure of the human PBAF ATP-dependent chromatin-remodeling complex preserved in negative stain by electron microscopy and have mapped the nucleosome binding site using two-dimensional (2D) image analysis. PBAF has an overall C-shaped architecture--with a larger density to which two smaller knobs are attached--surrounding a central cavity; one of these knobs appears to be flexible and occupies different positions in each of the structures determined. The 2D analysis of PBAF:nucleosome complexes indicates that the nucleosome binds in the central cavity.

Selectivity of chromatin-remodelling cofactors for ligand-activated transcription

An array of regulatory protein and multi-subunit cofactors has been identified that directs eukaryotic gene transcription. However, establishing the specific functions of various related cofactors has been difficult owing to the limitations inherent in assaying transcription in animals and cells indirectly. Here we describe, using an integrated chromatin-dependent reconstituted transcription reaction, the purification and identification of a multi-subunit cofactor (PBAF) that is necessary for ligand-dependent transactivation by nuclear hormone receptors. A highly related cofactor, human SWI/SNF, and the ISWI-containing chromatin-remodelling complex ACF both fail to potentiate transcription. We also show that transcriptional activation mediated by nuclear hormone receptors requires TATA-binding protein (TBP)-associated factors (TAFs) as well as the multi-subunit cofactors ARC/CRSP. These studies demonstrate functional selectivity amongst highly related complexes involved in gene regulation and help define a more complete set of factors and cofactors required to activate transcription.

Malignant melanoma: a literature review and case presentation

The authors present a case report of acral lentiginous malignant melanoma in a 77-year-old male. Melanoma is a rare but increasingly present malignant lesion of the lower extremity. It is the most common malignant neoplasm in blacks and is often misdiagnosed. Early, accurate diagnosis and biopsy of suspicious lesions is the cornerstone of treatment in order to decrease possible future morbidity and mortality. The authors discuss the clinical features, differential, diagnosis, predisposing factors, diagnosis, classification, and treatment of malignant melanoma.

Long-term efficacy of total SILASTIC implants: a subjective analysis

A retrospective analysis of the long-term efficacy of total SILASTIC implant arthroplasty performed before 1986 is presented. A total of 50 patients responded to subjective questionnaires regarding pain, function, complications, and overall patient satisfaction. The average age of the patients at the time of surgery was 55.1 years with an average follow-up of 13.4 years (range 10.7 to 16.9 years). Ninety-seven percent of patients reported relief from pain, and the overall success rating was 90.7%. Results were calculated based on a modification of the American Orthopaedic Foot and Ankle Society clinical rating system; the mean rating was 87.3. Attention must be directed at realigning the joint via appropriate osteotomies and soft tissue balancing procedures for increased success. Although radiographic deterioration of the implant was demonstrated in all implants, this deterioration did not correlate with patient satisfaction and should not be the sole criterion for implant removal. We conclude that total implant arthroplasty is a proven procedure for long-term relief of pain in selected patients with degenerative joint disease of the first metatarsophalangeal joint.

The dual-specificity phosphatase encoded by vaccinia virus, VH1, is essential for viral transcription in vivo and in vitro

The genetic complexity of vaccinia virus is such that as well as encoding its own transcription and replication machinery, it encodes two protein kinases and a protein phosphatase. The latter enzyme, designated VH1, is a prototype for the dual-specificity class of phosphatases. Here we report that the H1 phosphatase is encapsidated within vaccinia virions and describe the construction of a viral recombinant in which expression of the H1 gene is regulated by the presence or absence of isopropylthiogalactopyranoside (IPTG) in the culture medium. When expression of H1 is repressed, the number of viral particles produced is not compromised but the fraction of these particles which is infectious is significantly reduced. The lack of infectivity of the H1-deficient particles is specifically correlated with their inability to direct the transcription of early genes either in vitro or in vivo. A proximal role for the viral phosphatase in regulating the onset of viral gene expression is implied. Prominent among the encapsidated proteins found to be hyperphosphorylated in H1-deficient virions is the 11-kDa product of the F18 gene; this protein is the major DNA-binding component of the viral nucleoprotein complex. The ability of recombinant H1 phosphatase to reverse this hyperphosphorylation in permeabilized virions strengthens the conclusion that the F18 protein is a bona fide substrate for the H1 phosphatase.