Discovery of STRO-002, a Novel Homogeneous ADC Targeting Folate Receptor Alpha, for the Treatment of Ovarian and Endometrial Cancers

STRO-002 is a novel homogeneous folate receptor alpha (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 antitumor properties of STRO-002. STRO-002 was generated by conjugation of a novel cleavable 3-aminophenyl hemiasterlin linker-warhead (SC239) to the nonnatural amino acid para-azidomethyl-L-phenylalanine 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-glycoprotein 1 drug pump compared with other tubulin-targeting payloads. While STRO-002 lacks nonspecific cytotoxicity toward FolRα-negative cell lines, bystander killing of target negative cells was observed when cocultured 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 models. In addition, combination treatment with carboplatin or Avastin further increased STRO-002 efficacy in xenograft models. The potent and specific preclinical efficacy of STRO-002 supports clinical development of STRO-002 for treating patients with FolRα-expressing cancers, including ovarian, endometrial, and non-small cell lung cancer. Phase I dose escalation for STRO-002 is in progress in ovarian cancer and endometrial cancer patients (NCT03748186 and NCT05200364).

Abstract 5591: Anti-FolRα ADC STRO-002 induces immunogenic cell death (ICD) to enhance anti-tumor activity

STRO-002 is an antibody-drug conjugate (ADC) composed of an anti-folate receptor alpha (FolRα) antibody conjugated to a tubulin-targeting hemiasterlin (SC209) warhead via a cleavable linker currently in clinical studies for ovarian and endometrial cancer. FolRα exhibits minimal expression in normal tissues but is overexpressed in several cancers such as ovarian, endometrial, and NSCLC. We previously showed that STRO-002 induces in vitro hallmarks of immunogenic cell death (ICD), such as increased calreticulin exposure and HMGB1 and ATP release in a FolRα dependent manner. Follow-up vaccination studies with murine MC38 tumor cells engineered to express human FolRα (MC38-hFolRa) were performed to further explore the significance of STRO-002-induced ICD and protective immunity in vivo. A single vaccination with STRO-002 or SC209 in vitro-treated MC38-hFolRa cells in immunocompetent mice resulted in 60% of animals rejecting a subsequent rechallenge with living MC38-hFolRa cells, while relapse was observed in most control mice. Including an additional booster vaccination further increased the proportion ( 90%) of animals remaining tumor free after rechallenge. These results demonstrate that tumor cells pre-treated with STRO-002 or SC209 undergo potent immunogenic cell damage which can, in turn, mount protective immunity in vivo.

The ability of STRO-002 to induce ICD that elicits an effective and robust anti-tumor immune response may translate into several advantages in the clinic. Induction of ICD is likely to offer a complementary anti-tumor mechanism in combination therapy regimens such as with check point inhibitors or VEGF blockade. We have demonstrated that STRO-002 in combination with avelumab in MC38-hFolRα-bearing mice significantly enhanced efficacy and durable anti-tumor immunity. More recent rechallenge studies reveal that this combination treatment suppresses growth of not only MC38-hFolRα but also MC38 wild-type tumor cells, indicating formation of immunological memory with signs of epitope spreading. Additionally, co-administration of STRO-002 with anti-VEGF therapy demonstrates improved TGI compared to monotherapy (p < 0.0001) in human ovarian OV-90 tumors.

The potent cytotoxic and immunostimulatory properties of STRO-002 may also contribute to enhancing efficacy in indications with low FolRα expression. Such tumor types include endometrial and NSCLC, in which STRO-002 has displayed potent monotherapy activity in preclinical studies. In a panel of endometrial PDX models, STRO-002 was significantly efficacious in 9/17 (53%) FolRα-positive tumors, including models with moderate and weak FolRα levels. In a NSCLC PDX model, a single dose of STRO-002 induced tumor regression and suppressed tumor growth for up to three months post-dose. In conclusion, the induction of ICD by STRO-002 has potential to increase anti-tumor activity in cancers with low FolRα expression.

Citation Format: Cristina L. Abrahams, Andrew McGeehan, Jennifer Smith, Robert Yuan, Kshama A. Doshi, Sihong Zhou, Xiaofan Li, Cuong Tran, Gang Yin, Arturo Molina, Kristin Bedard, Trevor Hallam. Anti-FolRα ADC STRO-002 induces immunogenic cell death (ICD) to enhance anti-tumor activity [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 5591.

Abstract 2250: STRO-002, an anti-FolRαADC, demonstrates immune-modulating properties and potentiates PD-L1 blockade

There is growing evidence that tumor-targeted cytotoxins can also enhance anti-tumor immunity by inducing immunogenic cell death (ICD) in tumor cells and promoting recruitment of immune effector cells. We sought to investigate the immune stimulating potential of STRO-002, an antibody drug conjugate (ADC) composed of an anti-Folate receptor alpha (FolRα) antibody conjugated to a tubulin-targeting hemiasterlin warhead via a cleavable linker. FolRα is a single chain glycosylphosphatidylinositol-anchored membrane receptor glycoprotein with minimal expression in normal tissues. Its overexpression in several cancer indications has been described, including in ovarian, endometrial, non-small cell lung carcinoma (NSCLC), and triple negative breast cancer (TNBC), thus making it an ideal ADC target. We have previously demonstrated potent in vitro and in vivo activity of STRO-002 in several FolRα expressing models. Here we show that the hemiasterlin warhead, SC209, and STRO-002 ADC induced ICD in vitro as evidenced by presentation of cell-surface calreticulin and release of HMGB1 and ATP. As a result of ICD, STRO-002 treated FolRα positive cancer cells induced antigen-dependent monocyte activation, as well as, increased phagocytic activity in PBMCs co-cultured with tumor cells. To determine if these immunogenic properties could improve therapeutic efficacy, we evaluated STRO-002 in combination with the immune checkpoint inhibitor Avelumab (anti-PD-L1) in a mouse syngeneic MC38 model engineered to express human FolRα (MC38-hFolRα). Results showed that STRO-002 and Avelumab alone inhibited tumor growth and could induce complete responses (e.g. no palpable tumors) at low frequency (< 15%), while co-administration of STRO-002 and Avelumab significantly enhanced efficacy leading to complete response in the majority of animals. Furthermore, when animals that initially achieved complete response were re-challenged with MC38-hFolRα cells, they showed durable anti-tumor immunity, indicating formation of immunological memory. Immunohistochemical analysis conducted seven days after treatment revealed a significant increase in tumor-infiltrating cytotoxic CD8+ T cells in animals treated with combination of STRO-002 + Avelumab versus either monotherapy. Cumulatively, these results suggest that STRO-002 synergizes with Avelumab to enhance anti-tumor response by inducing ICD in tumor cells, which in turn, promote T cell recruitment. These data support the rationale for combining STRO-002 with immune checkpoint inhibitors to potentially enhance their clinical efficacy.

Citation Format: Millicent Embry, Sihong Zhou, Christine Cheng, Janice Yu, Cristina L. Abrahams, Xioafan Li, Jeff Hanson, Cuong Tran, Gang Yin, Shamim Ahmad, Krishna Bajjuri, Venita DeAlmeida, Mark Lupher, Trevor Hallam. STRO-002, an anti-FolRαADC, demonstrates immune-modulating properties and potentiates PD-L1 blockade [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 2250.

Targeting CD74 in multiple myeloma with the novel, site-specific antibody-drug conjugate STRO-001

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.

Abstract 4809: Enhanced activity of aflibercept compared to bevacizumab in a patient-derived xenograft colorectal cancer model is associated with reduced tumor vascularity in humanized VEGF mice

VEGF inhibitors aflibercept (ziv-aflibercept in USA) and bevacizumab target vascular endothelial growth factor A (VEGF-A), and are approved for the 2nd line treatment of metastatic colorectal cancer (CRC) in combination with distinct chemotherapy regimens and after different 1st line therapies. Although both aflibercept and bevacizumab bind to VEGF-A (herein referred to as VEGF), aflibercept blocks additional VEGF family ligands PlGF and VEGF-B, which bind VEGF receptor 1 (VEGFR1), whereas bevacizumab does not. We recently utilized patient-derived xenograft (PDX) models of CRC to compare the pharmacological activity of aflibercept and bevacizumab. In a screening study in nude mice, aflibercept showed more potent anti-tumor activity in 39/48 CRC PDX compared to bevacizumab (Chiron M. et al, AACR-NCI-EORTC 2013). In these PDX, tumor levels of tumor-derived human VEGF (hVEGF) were much higher than host murine VEGF (mVEGF), however, a potential limitation was that bevacizumab does not bind mVEGF. To further rule out any contribution from mVEGF and to determine whether tumor cell expression of VEGFR1 played a role in the differential responses, we compared the effects of aflibercept and bevacizumab on selected PDX CRC tumors in SCID mice genetically engineered to express hVEGF from the mVEGF locus. In humanized VEGF mice, aflibercept was more potent in 3/7 PDX compared with bevacizumab but equivalent in 4/7 PDX. Tumors in PDX IMM-Colo-0018 and IMM-Colo-0010, with high and low VEGFR1 expression, respectively, regressed in response to aflibercept while bevacizumab resulted in growth stasis. PDX model CXF280 with high VEGFR1 expression, consistently showed significantly greater response to aflibercept in comparison to bevacizumab. In contrast, PDX model CFX260, also with high VEGFR1 expression, responded equally well to both drugs. There was no CRC PDX in humanized VEGF mice in which the activity of bevacizumab was greater than aflibercept. Immunohistochemistry analysis revealed that improved efficacy of aflibercept treatment over bevacizumab was associated with a greater reduction of tumor vessel density and sprouting in CFX280 tumors. Cumulatively, these results suggest that aflibercept is more potent than bevacizumab in certain PDX CRC tumor models. In this limited subset of PDX CRC models, differences in pharmacological activity observed between aflibercept and bevacizumab in humanized VEGF mice do not appear to correlate with tumor cell expression of VEGFR1, but instead are associated with a greater reduction of tumor blood vessels. We are further investigating the role of VEGFR1 and other factors that may explain enhanced anti-tumor efficacy of aflibercept in PDX CRC models in humanized mice. The results suggest that CRC patient subsets may exist where aflibercept is more active than therapies targeting only VEGF-A.

Citation Format: Cristina L. Abrahams, Laura D. Winters, Baosheng Li, Asma A. Parveen, Tatjana Kloss, Rebecca G. Bagley, Donald A. Bergstrom, Gavin Thurston, Marielle Chiron. Enhanced activity of aflibercept compared to bevacizumab in a patient-derived xenograft colorectal cancer model is associated with reduced tumor vascularity in humanized VEGF mice. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4809. doi:10.1158/1538-7445.AM2014-4809

Functional consequences of Wnt-induced dishevelled 2 phosphorylation in canonical and noncanonical Wnt signaling

Dishevelled (Dvl) proteins are intracellular effectors of Wnt signaling that have essential roles in both canonical and noncanonical Wnt pathways. It has long been known that Wnts stimulate Dvl phosphorylation, but relatively little is known about its functional significance. We have previously reported that both Wnt3a and Wnt5a induce Dvl2 phosphorylation that is associated with an electrophoretic mobility shift and loss of recognition by monoclonal antibody 10B5. In the present study, we mapped the 10B5 epitope to a 16-amino acid segment of human Dvl2 (residues 594-609) that contains four Ser/Thr residues. Alanine substitution of these residues (P4m) eliminated the mobility shift induced by either Wnt3a or Wnt5a. The Dvl2 P4m mutant showed a modest increase in canonical Wnt/β-catenin signaling activity relative to wild type. Consistent with this finding, Dvl2 4Pm preferentially localized to cytoplasmic puncta. In contrast to wild-type Dvl2, however, the P4m mutant was unable to rescue Wnt3a-dependent neurite outgrowth in TC-32 cells following suppression of endogenous Dvl2/3. Earlier work has implicated casein kinase 1δ/ε as responsible for the Dvl mobility shift, and a CK1δ in vitro kinase assay confirmed that Ser(594), Thr(595), and Ser(597) of Dvl2 are CK1 targets. Alanine substitution of these three residues was sufficient to abrogate the Wnt-dependent mobility shift. Thus, we have identified a cluster of Ser/Thr residues in the C-terminal domain of Dvl2 that are Wnt-induced phosphorylation (WIP) sites. Our results indicate that phosphorylation at the WIP sites reduces Dvl accumulation in puncta and attenuates β-catenin signaling, whereas it enables noncanonical signaling that is required for neurite outgrowth.

Abstract 4271: Combination of VEGF Trap (Aflibercept) with docetaxel, but not with gemcitabine, is efficacious in tumors resistant to anti-VEGF therapy

Increased anti-tumor efficacy is often observed when VEGF blockade is combined with certain chemotherapy agents. However, the mechanism by which combination treatment with certain agents, but not others, results in enhanced anti-tumor effects is poorly understood. In this study, we compared the anti-tumor effects of VEGF Trap in combination with either docetaxel or gemcitabine on HT1080, a human fibrosarcoma tumor line that is resistant to anti-VEGF treatment. Immunodeficient SCID mice bearing subcutaneous HT1080 tumors (size ∼150 mm3) were treated with either control protein plus vehicle, VEGF Trap, chemotherapy agent (docetaxel or gemcitabine), or the combination. Following three treatments administered every 3-4 days, tumors were harvested either one day after the final dose or allowed to grow and harvested when the average size of the treatment group reached ∼500mm3. We found that combination treatment with VEGF Trap plus docetaxel was significantly more effective compared to single agents, while there was no added benefit of combining VEGF Trap with gemcitabine. Although treatment with chemotherapy agents alone and in combination with VEGF Trap both initially regressed tumor size, only the combination with docetaxel was effective at delaying long-term tumor re-growth. Immunohistochemical analysis of tumors showed that both combination regimens produced a dramatic decrease in the number of proliferating tumor cells. However, only VEGF Trap and docetaxel combination treatment resulted in pruned tumor vessels with less branching, whereas VEGF Trap plus gemcitabine had minimal effect on tumor vessel morphology. Furthermore, VEGF trap plus docetaxel produced an increase in TUNEL-positive cells associated with the tumor vasculature and a marked decrease in tumor pericytes. These results suggest that in tumors resistant to VEGF blockade, taxanes (docetaxel) can sensitize the vasculature to anti-VEGF treatment (aflibercept), thus resulting in enhanced effects on the tumor vessels and improved anti-tumor efficacy. Further understanding of the mechanistic basis of different combination regimens may allow improvement of current strategies to treat tumors resistant to antiangiogenic therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4271. doi:10.1158/1538-7445.AM2011-4271