The development of antibody drug conjugates has provided enhanced potency to tumor-targeting antibodies by the addition of highly potent payloads. In the case of trastuzumab-DM1 (T-DM1), approved for the treatment of metastatic breast cancer, the addition of mertansine (DM1) to trastuzumab substantially increased progression-free survival. Despite these improvements, most patients eventually relapse due to complex mechanisms of resistance often associated with small molecule chemotherapeutics. Therefore, identifying payloads with different mechanisms of action (MOA) is critical for increasing the efficacy of targeted therapeutics and ultimately improving patient outcomes. To evaluate payloads with different MOA, deBouganin, a deimmunized plant toxin that inhibits protein synthesis, was conjugated to trastuzumab and compared with T-DM1 both in vitro and in vivo. The trastuzumab-deBouganin conjugate (T-deB) demonstrated greater potency in vitro against most cells lines with high levels of Her2 expression. In addition, T-deB, unlike T-DM1, was unaffected by inhibitors of multidrug resistance, Bcl-2-mediated resistance, or Her2-Her3 dimerization. Contrary to T-DM1 that showed only minimal cytotoxicity, T-deB was highly potent in vitro against tumor cells with cancer stem cell properties. Overall, the results demonstrate the potency and efficacy of deBouganin and emphasize the importance of using payloads with different MOAs. The data suggest that deBouganin could be a highly effective against tumor cell phenotypes not being addressed by current antibody drug conjugate formats and thereby provide prolonged clinical benefit.

VB6-845 is a recombinant immunotoxin comprised of deBouganin (a de-immunized plant toxin) genetically linked to an epithelial cell adhesion molecule (EpCAM)-targeting humanized Fab fragment (4D5MOCB). EpCAM is highly expressed on a wide range of epithelial tumors but has limited expression on most normal epithelia and therefore represents an excellent target for immunotherapy. A comprehensive preclinical evaluation was performed to determine the safety and suitability of VB6-845 as a systemically administered drug for the treatment of solid tumors. Efficacy studies in mice demonstrated that VB6-845 specifically and potently targeted EpCAM-positive tumors. In a dose-ranging study in Sprague-Dawley rats, single doses of VB6-845 were well-tolerated resulting in a no-observable adverse effect level (NOAEL) of 100 mg/kg whereas repeated doses of VB6-845 resulted in vascular leak-associated symptoms particularly at higher dose levels. However, much higher doses in Cynomolgus monkeys were well-tolerated when given as a 3-hour infusion mimicking the intended route of administration in the clinic. In addition, VB6-845 proved to be minimally immunogenic in monkeys. The toxicological data obtained in Cynomolgus monkeys indicated an excellent safety profile with a NOAEL value of 30 mg/kg (equivalent to a 10 mg/kg dose in humans). These results are supportive of an exploratory Phase I trial.

Antibody drug conjugates (ADC), comprised of highly potent small molecule payloads chemically conjugated to a full-length antibody, represent a growing class of therapeutic agents. The targeting of cytotoxic payloads via the specificity and selectivity of the antibody has led to substantial clinical benefits. However, ADC potency can be altered by mechanisms of resistance such as overexpression of efflux pumps or anti-apoptotic proteins. DeBouganin is a de-immunized variant of bouganin, a ribosome-inactivating protein (RIP) that blocks protein synthesis, thereby leading to apoptosis. When conjugated to trastuzumab (T-deB), deBouganin was more potent than ado-trastuzumab-emtansine (T-DM1) and unaffected by resistance mechanisms to which DM1 is susceptible. To further highlight the differentiating mechanism of action of deBouganin, HCC1419 and BT-474 tumor cells that survived T-DM1 or trastuzumab-MMAE (T-MMAE) treatment were treated with an anti-HER2 C6.5 diabody-deBouganin fusion protein or T-deB. C6.5 diabody-deBouganin and T-deB were potent against HCC1419 and BT-474 cells that were resistant to T-DM1 or T-MMAE killing. The resistant phenotype involved MDR pumps, Bcl-2 family members, and the presence of additional unknown pathways. Overall, the data suggest that deBouganin is effective against tumor cell resistance mechanisms selected in response to ADCs composed of anti-microtubule payloads.

DeBouganin (deB) is a de-immunized form of bouganin, a Ribosome Inactivating Protein (RIP) that when internalized blocks protein synthesis thereby leading to cell death. When conjugated to trastuzumab (T-deB) or genetically attached to the C6.5 diabody, deBouganin was more potent than DM1 and unaffected by mechanisms of resistance to which DM1 is susceptible. To further highlight the differentiating mechanism of action of deBouganin, HCC1419 and BT-474 tumor cells that survived T-DM1 or trastuzumab-MMAE (T-MMAE) treatment in vitro were re-exposed to T-DM1, T-MMAE, or treated with T-deB or an anti-HER2 C6.5 diabody-deBouganin fusion protein. C6.5 diabody-deBouganin and T-deB were potent against HCC1419 and BT-474 cells surviving T-DM1 or T-MMAE treatment. However, the surviving cell populations were resistant to T-DM1, T-MMAE, DM1, MMAE and taxol treatment. In addition, cross-resistance was seen against trastuzumab-duocarmycin which contains a payload with a cell cycle independent mechanism of action. The contribution of multi-drug resistance, Bcl-2 family members and other survival pathways accounting for the resistant phenotype will be discussed. Overall, the data suggest that treatment with chemotherapeutics or ADCs comprised of small molecule compounds such as anti-microtubule agents, can lead to the outgrowth of tumor cells resistant to similar agents. In contrast, antibodies and antibody fragments armed with deBouganin can overcome these mechanisms of resistance and may therefore represent a more effective treatment option. Citation Format: Shilpa Chooniedass, Rachelle L. Dillon, Arjune Premsukh, Gregory P. Adams, Glen C. MacDonald, Jeannick Cizeau. Trastuzumab and C6.5 diabody armed with deBouganin overcome drug resistance to ADCs comprised of anti-microtubule agents [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 79. doi:10.1158/1538-7445.AM2017-79

VB4-845 is a Targeted Protein Therapeutic (TPT) comprised of a scFv fragment specific for the Epithelial Cell Adhesion Molecule (EpCAM) genetically fused to a truncated form of Pseudomonas exotoxin A (ETA), via a flexible linker. Due to the inherent immunogenicity of the ETA moiety, VB4-845 is only used for the treatment of loco-regionally accessible tumors and is currently in a Phase III clinical trial for the treatment of high grade, non-muscle invasive bladder cancer. In a Phase I study in late stage squamous cell carcinoma of the head and neck, direct injection of VB4-845 led to shrinkage of the principal injected tumor, as well as non-targeted tumors in some patients, suggesting the activation of a T cell-mediated anti-tumor response through cross-priming. Only cells undergoing immunogenic cell death, or ICD, are capable of stimulating cross-priming. ICD is characterized by distinct “eat-me” signals such as the release of ATP and HMGB1, and cell surface translocation of calreticulin, an endoplasmic reticulum chaperone protein. To evaluate whether VB4-845-mediated cell killing results in ICD events, treated tumor cell lines were assessed for these distinct signaling molecules. In vitro studies using flow cytometry and ELISA showed both translocation of calreticulin and release of ATP and HMGB1 by VB4-845-treated tumor cells. Immune T cell activation via an ICD pathway suggests a complimentary outcome when combined with checkpoint inhibitors. A study was performed in PDX tumor-bearing NOG mice reconstituted with a human immune system to assess the combination of intratumoral injection of VB4-845 with an anti-PD1 antibody, Nivolumab, given i.p. VB4-845 showed growth suppression of the injected tumor whereas the growth delay of the contralateral, non-injected tumor was more pronounced with the addition of Nivolumab. In summary, the data presented suggests that VB4-845 mediates tumor cell killing by an ICD pathway and that the resulting cross-priming effect can enhance the anti-tumoral activity of immune checkpoint inhibitors. Citation Format: Rachelle Lee Dillon, Shilpa Chooniedass, Arjune Premsukh, Glen MacDonald, Jeannick Cizeau, Gregory A. Adams. VB4-845 tumor cell killing in a combination study with the anti-PD-1, Nivolumab [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 614. doi:10.1158/1538-7445.AM2017-614