Antibody drug conjugates (ADCs) are a therapeutic class offering promise for cancer therapy. The attachment of cytotoxic drugs to antibodies can result in an effective therapy with better safety potential than nontargeted cytotoxics. To understand the role of conjugation site, we developed an enzymatic method for site-specific antibody drug conjugation using microbial transglutaminase. This allowed us to attach diverse compounds at multiple positions and investigate how the site influences stability, toxicity, and efficacy. We show that the conjugation site has significant impact on ADC stability and pharmacokinetics in a species-dependent manner. These differences can be directly attributed to the position of the linkage rather than the chemical instability, as was observed with a maleimide linkage. With this method, it is possible to produce homogeneous ADCs and tune their properties to maximize the therapeutic window.

The systemic stability of the antibody−drug linker is crucial for delivery of an intact antibody−drug conjugate (ADC) to target-expressing tumors. Linkers stable in circulation but readily processed in the target cell are necessary for both safety and potency of the delivered conjugate. Here, we report a range of stabilities for an auristatin-based payload site-specifically attached through a cleavable valine-citrulline-p-aminobenzylcarbamate (VC-PABC) linker across various sites on an antibody. We demonstrate that the conjugation site plays an important role in determining VC-PABC linker stability in mouse plasma, and that the stability of the linker positively correlates with ADC cytotoxic potency both in vitro and in vivo. Furthermore, we show that the VC-PABC cleavage in mouse plasma is not mediated by Cathepsin B, the protease thought to be primarily responsible for linker processing in the lysosomal degradation pathway. Although the VC-PABC cleavage is not detected in primate plasma in vitro, linker stabilization in the mouse is an essential prerequisite for designing successful efficacy and safety studies in rodents during preclinical stages of ADC programs. The divergence of linker metabolism in mouse plasma and its intracellular cleavage offers an opportunity for linker optimization in the circulation without compromising its efficient payload release in the target cell.

STRO-001 is a site-specific, predominantly single-species, fully human, aglycosylated anti-CD74 antibody-drug conjugate incorporating a non-cleavable linker-maytansinoid warhead with a drug-antibody ratio of 2 which was produced by a novel cell-free antibody synthesis platform. We examined the potential pharmacodynamics and anti-tumor effects of STRO-001 in multiple myeloma (MM). CD74 expression was assessed in MM cell lines and primary bone marrow (BM) MM biopsies. CD74 mRNA was detectable in CD138+ enriched plasma cells from 100% (892/892) of patients with newly diagnosed MM. Immunohistochemistry confirmed CD74 expression in 35/36 BM biopsies from patients with newly diagnosed and relapsed/refractory MM. Cytotoxicity assays demonstrated nanomolar STRO-001 potency in 4/6 MM cell lines. In ARP-1 and MM.1S tumor-bearing mice, repeat STRO-001 dosing provided significant antitumor activity with eradication of malignant hCD138+ BM plasma cells and prolonged survival. In a luciferase-expressing MM.1S xenograft model, dose-dependent STRO-001 efficacy was confirmed using bioluminescent imaging and BM tumor burden quantification. Consistent with the intended pharmacodynamic effect, STRO-001 induced dose-responsive, reversible B-cell and monocyte depletion in cynomolgus monkeys, up to a maximum tolerated 10 mg/kg, with no evidence of off-target toxicity. Collectively, these data suggest that STRO-001 is a promising therapeutic agent for the treatment of MM.

The ability to exogenously impose targeted epigenetic changes in the genome represents an attractive route for the simulation of genomic de novo epigenetic events characteristic of some diseases and for the study of their downstream effects and also provides a potential therapeutic approach for the heritable repression of selected genes. Here we demonstrate for the first time the ability of zinc finger peptides to deliver DNA cytosine methylation in vivo to a genomic integrated target promoter when expressed as fusions with a mutant prokaryotic DNA cytosine methyltransferase enzyme, thus mimicking cellular genomic de novo methylation events and allowing a direct analysis of the mechanics of de novo DNA methylation-mediated gene silencing at a genomic locus. We show that targeted methylation leads to gene silencing via the initiation of a repressive chromatin signature at the targeted genomic locus. This repression is maintained after the loss of targeted methyltransferase enzyme from the cell, confirming epigenetic maintenance purely through the action of cellular enzymes. The inherited DNA methylation pattern is restricted only to targeted sites, suggesting that the establishment of repressive chromatin structure does not drive further de novo DNA methylation in this system. As well as demonstrating the potential of these enzymes as tools for the exogenous, heritable control of cellular gene expression, this work also provides the most definitive confirmation to date for a transcriptionally repressive role for de novo DNA methylation in the cell and lends some weight to the hypothesis that the aberrant methylation associated with certain diseases may well be a cause rather than a consequence of transcriptional gene repression.

Folate receptor alpha (FolRα) is a glycosylphosphatidylinositol linked cell-surface glycoprotein that is widely expressed in serous and epithelial ovarian cancer, endometrial adenocarcinoma, non-small cell lung cancer and triple negative breast cancer. In contrast, FolRα expression is highly restricted on normal tissues, making it a highly promising target for cancer therapy using antibody drug conjugates (ADCs). We have designed a novel, FolRα-targeting ADC, STRO-002, with potent cytotoxic activity on FolRα expressing tumors in vitro and in vivo, including in cells with low expression levels (~0.2 million copies/cell) of FolRα. STRO-002 contains the anti-FolRa human IgG1 antibody (SP8166) conjugated to a proprietary cleavable drug-linker (SC239). SC239 contains a tubulin-targeting 3-aminophenyl hemiasterlin warhead, SC209, which has potent cytotoxic activity and is a weak substrate for efflux pumps. SP8166 was discovered and optimized using a Fab ribosome display selection and screening platform based on Sutro's Xpress CF+TM system. Four non-natural amino acid p-azidomethyl phenylalanine (pAMF) residues are incorporated into SP8166 at two defined sites on each heavy chain. These sites were selected based on optimal stability and activity in vitro and in vivo. The SC239 drug-linker is conjugated via a cleavable valine citrulline p-aminobenzyl carbamate linker functionalized with dibenzocyclooctyne (DBCO).  The rapid and selective reaction of DBCO and pAMF results in a well-defined, homogeneous ADC with a drug-antibody ratio (DAR) of ~4. STRO-002 has potent cytotoxic activity (0.1-3 nM) on multiple FolRα-positive ovarian cancer cell lines in vitro and demonstrates strong anti-tumor response in KB, Igrov1 and OvCAR3 xenograft models in vivo. On Igrov1 xenografts, STRO-002 exhibits dose-dependent tumor growth inhibition starting at a single dose as low as 2.5 mg/kg. Evaluation of in vivo activity of STRO-002 in additional xenograft and PDX models, as well as in combination studies with chemotherapeutic agents is ongoing. Data from exploratory safety studies of STRO-002 in cynomolgus monkey and SC209 (active catabolite) in rats show a favorable safety profile. Our data suggests that STRO-002 is a promising clinical candidate for ovarian cancer, including tumors with low expression levels of FolRα, and IND enabling studies are currently being conducted. Citation Format: Xiaofan Li, Cristina Abrahams, Sihong Zhou, Stellanie Krimm, Robert Henningsen, Heather Stephenson, Jeffrey Hanson, Mary Rose Masikat, Krishna Bajjuri, Tyler Heibeck, Cuong Tran, Gang Yin, James Zawada, Ganapathy Sarma, Joy Chen, Maureen Bruhns, Willy Solis, Alexander Steiner, Adam Galan, Toni Kline, Ryan Stafford, Alice Yam, Venita I. De Almeida, Mark Lupher, Trevor Hallam. Discovery and activity of STRO-002, a novel ADC targeting folate receptor alpha for ovarian and endometrial 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 1782.

OBJECTIVES: Folate receptor alpha (FolRα) is a cell-surface glycoprotein, highly expressed in ovarian and endometrial adenocarcinoma, and thus a promising target for cancer therapy using antibody drug conjugates (ADCs). Most ADCs currently in development are generated by random attachment of the cytotoxic payload to the antibody and result in a heterogeneous mixture, comprised of many different forms that are likely to vary in stability and activity, and therefore may be suboptimal therapeutic agents. We have employed an E. coli cell-free expression system (XpressCFTM) and site-specific conjugation technology, to generate STRO-002, a novel homogenous FolRα-targeting ADC. STRO-002 was optimized by selection of the antibody, drug-linker, conjugation site and drug-antibody ratio (DAR) that conferred the best pharmacological properties. We have conducted preclinical studies to evaluate the stability of STRO-002 and characterize the pharmacological properties of the cytotoxic metabolite SC209. In vitro cytotoxicity assays and in vivo efficacy studies were conducted to evaluate the activity of STRO-002 in multiple ovarian cancer cell lines and xenografts. IND enabling toxicology studies were conducted to determine the safety profiles for STRO-002 and its metabolite SC209 in cynomolgous monkeys and rats, respectively. RESULTS: Based on optimization studies, the anti-FolRα human IgG1 antibody (H01/SP8166) conjugated to a proprietary cleavable drug-linker (SC239) was selected for the lead ADC STRO-002. SC239 contains a tubulin-targeting 3-aminophenyl hemiasterlin warhead, SC209, which has potent cytotoxic activity. Based on most favorable anti-tumor activity, positions 180 and 404 on each heavy chain were selected for conjugation of SC239 to SP8166 to yield an ADC with DAR of ~ 4. The drug-linkage in STRO-002 is highly stable and the released warhead, SC209, is a very weak substrate for cellular drug-resistance efflux pumps and is cleared rapidly from plasma. STRO-002 has potent but highly specific cytotoxic activity (0.1-3 nM) on multiple FolRα-positive ovarian cancer cell lines in vitro and anti-tumor efficacy in ovarian xenograft models. STRO-002 exhibits dose-dependent tumor growth inhibition in Igrov-1 tumor xenografts at a single dose and complete regression is achieved in Igrov-1 and OVCAR-3 tumors with a single dose at 10 and 5 mg/kg, respectively. In addition, administration of STRO-002 in combination with carboplatin confers added benefit in efficacy in Igrov-1 tumors. Toxicology studies show favorable safety profiles for STRO-002 and SC209. The main toxicity finding in monkeys dosed up to 9 mg/kg consists of reversible hematopoietic/lymphoid tissue toxicity, which is considered antigen-independent and is consistent with the anti-proliferative effects of SC209 observed in single-dose toxicology studies in rats. No evidence of ocular toxicity due to SC209 were observed in either species. CONCLUSIONS: STRO-002 is a highly specific FolRα targeting ADC with minimal drug moiety release in circulation and the potential for an improved safety and activity profile, and a reduced risk of tumor drug resistance. Our data supports the advancement of STRO-002 to the clinic as a potential treatment of FolRα expressing malignancies such as ovarian cancer. Citation Format: Cristina Abrahams, Stellanie Krimm, Xiaofan Li, Sihong Zhou, Jeffrey Hanson, Mary Rose Masikat, Krishna Bajjuri, Tyler Heibeck, Dharti Kothari, Abigail Yu, Robert Henningsen, Cuong Tran, Gang Yin, James Zawada, Julie Hang, Maureen Bruhns, Willy Solis, Alexander Steiner, Adam Galan, Toni Kline, Ryan Stafford, Alice Yam, Venita I. De Almeida, Mark Lupher, Jr., Trevor Hallam. PRECLINICAL ACTIVITY AND SAFETY OF STRO-002, A NOVEL ADC TARGETING FOLATE RECEPTOR ALPHA FOR OVARIAN AND ENDOMETRIAL CANCER [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-090.

STRO-002 is a novel homogeneous FolRα targeting antibody drug conjugate (ADC), currently being investigated in the clinic as a treatment for ovarian and endometrial cancers. Here we describe the discovery, optimization, and anti-tumor properties of STRO-002. STRO-002 was generated by conjugation of a novel cleavable 3-aminophenyl hemiasterlin linker-warhead (SC239) to the non-natural amino acid para-azidomethyl-L-phenylalanine (pAMF) incorporated at specific positions within a high affinity anti-FolRα antibody using Sutro’s XpressCF+™, which resulted in a homogeneous ADC with a drug-antibody ratio (DAR) of 4. STRO-002 binds to FolRα with high affinity, internalizes rapidly into target positive cells, and releases the tubulin-targeting cytotoxin 3-aminophenyl hemiasterlin (SC209). SC209 has reduced potential for drug efflux via P-gp drug pumps compared to other tubulin-targeting payloads. While STRO-002 lacks non-specific cytotoxicity toward FolRα-negative cell lines, bystander killing of target negative cells was observed when co-cultured with target positive cells. STRO-002 is stable in circulation with no change in DAR for up to 21 days and has a half-life of 6.4 days in mice. A single dose of STRO-002 induced significant tumor growth inhibition in FolRα expressing xenograft models and patient derived xenograft (PDX) models. In addition, combination treatment with carboplatin or Avastin further increased STRO-002 efficacy in xenograft models. The potent and specific pre-clinical efficacy of STRO-002 supports clinical development of STRO-002 for treating patients with FolRα-expressing cancers including ovarian, endometrial, and NSCLC. Phase I dose escalation for STRO-002 is in progress in ovarian cancer patients (NCT03748186).

CD74, also known as HLA-DR-associated invariant chain, is a type II transmembrane glycoprotein highly expressed in many B-cell malignancies. The limited expression of CD74 in normal tissues suggests it may be a suitable ADC target for these tumor types. Accordingly, we engineered an anti-CD74 human IgG1 antibody (SP7219) using novel Fab-based ribosome display methods. The selected Fabs were readily reformatted and directly screened as IgGs using Sutro's unique high-throughput, cell-free protein synthesis platform, Xpress CFTM. We then developed novel, potent ADCs, SP7676 and SP7675 (STRO-001), comprised of our lead antibody (SP7219) conjugated to non-cleavable DBCO-maytansinoid linker-warheads with an average drug-antibody ratios (DAR) of 2. We used site-specific conjugation technology which results in a high degree of homogeneity characterized by the drug linker covalently binding to a single defined site. The sites for conjugation were selected based on highest cell killing activity and stablity in vitro and in vivo. Both ADCs demonstrate potent cell killing activity across multiple B-cell tumor lines in vitro, and anti-tumor activity in preclinical multiple myeloma xenograft models. In vitro cytotoxicity assays show nanomolar potency of STRO-001 in four MM cell lines: Mc/CAR (IC50 0.8 nM), MM.1S (IC50 10-11 nM), U266B1 (IC50 8.5 -9.3 nM), and ARP-1 (IC50 4.3-22 nM). CD74 cell surface expression is required for ADC anti-proliferative effect but the expression level does not seem to correlate with in vitro potency. SP7676 elicited a robust anti-tumor response in the ANBL-6 multiple myeloma xenograft model. Durable regressions were observed in all mice at ≥ 3 mg/kg, with equivalent efficacy (regression) at 3 mg/kg (every 3 days x5) and 10 mg/kg (every 3 days x5 or weekly x3). SP7676 also elicited a clear survival benefit in a disseminated multiple myeloma CAG xenograft model starting at 1mg/kg every 3 days x 5 doses. Similarly, both SP7676 and STRO-001 inhibited the formation of internal visceral tumors in the ARP-1 xenograft model after 3 weekly doses of 3 mg/kg. Evaluation of our lead candidate, STRO-001 in additional MM cell lines and primary patient samples is planned. The tolerability of STRO-001 in non-human primates is under evaluation. STRO-001 was administered to cynomolgous monkeys in an exploratory dose-escalating study up to 30 mg/kg x 2 doses on Day 1 and 15. STRO-001 reduces normal B-cell populations at ≥1 mg/kg after a single dose, providing pharmacodynamic evidence of B-cell targeting while other hematopoietic lineages are mostly affected only at the highest dose studied. Anticipated hematologic toxicities were readily reversible at 1, 3 and 10 mg/kg and target organs of interest were identified. Based on these encouraging data, STRO-001 is advancing to IND-enabling studies for the treatment of CD74 expressing multiple myeloma and other B-cell malignancies. Disclosures Abrahams: Sutro Biopharma: Employment. Li:Sutro Biopharma: Employment. Yu:Sutro Biopharma: Employment. Krimm:Sutro Biopharma: Employment. Kahana:Celgene: Employment. Narla:Celgene: Employment. Schwartz:Celgene: Employment. Boylan:Celgene: Employment. Hoffmann:Sutro Biopharma: Employment. Steiner:Sutro Biopharma: Employment. Zawada:Sutro Biopharma: Employment. Stephenson:Sutro Biopharma: Employment. Bruhns:Sutro Biopharma: Employment. DeAlmeida:Sutro Biopharma: Employment. Matheny:Sutro Biopharma: Employment. Bussell:Sutro Biopharma: Employment. Galan:Sutro Biopharma: Employment. Kline:Sutro Biopharma: Employment. Vasquez:Sutro Biopharma: Employment. Yam:Sutro Biopharma: Employment. Stafford:Sutro Biopharma: Employment. Heinsohn:Sutro Biopharma: Employment. Sato:Sutro Biopharma: Employment. Molina:Sutro Biopharma: Employment. Hallam:Sutro Biopharma: Employment. Lupher:Sutro Biopharma: Employment.