Use of pyrrolobenzodiazepines and related covalent-binding DNA-interactive molecules as ADC payloads: Is mechanism related to systemic toxicity?

Development of a Novel DNA Mono-alkylator Platform for Antibody-Drug Conjugates

Although microtubule inhibitors (MTI) remain a therapeutically valuable payload option for antibody-drug conjugates (ADC), some cancers do not respond to MTI-based ADCs. Efforts to fill this therapeutic gap have led to a recent expansion of the ADC payload "toolbox" to include payloads with novel mechanisms of action such as topoisomerase inhibition and DNA cross-linking. We present here the development of a novel DNA mono-alkylator ADC platform that exhibits sustained tumor growth suppression at single doses in MTI-resistant tumors and is well tolerated in the rat upon repeat dosing. A phosphoramidate prodrug of the payload enables low ADC aggregation even at drug-to-antibody ratios of 5:1 while still delivering a bystander-capable payload that is effective in multidrug resistant (MDR)-overexpressing cell lines. The platform was comparable in xenograft studies to the clinical benchmark DNA mono-alkylator ADC platform DGN459 but with a significantly better tolerability profile in rats. Thus, the activity and tolerability profile of this new platform make it a viable option for the development of ADCs.

MTX-13, a Novel PTK7-Directed Antibody-Drug Conjugate with Widened Therapeutic Index Shows Sustained Tumor Regressions for a Broader Spectrum of PTK7-Positive Tumors

Protein tyrosine kinase 7 (PTK7) is a Wnt signaling pathway protein implicated in cancer development and metastasis. When using a potent microtubule inhibitor (Aur0101), PTK7-targeting antibody-drug conjugate (ADC), h6M24-vc0101 (PF-06647020/cofetuzumab pelidotin) is efficacious only in limited tumor types with low response rates in a phase I trial. To improve patient response and to expand responding tumor types, we designed MTX-13, a PTK7-targeting ADC consisting of a novel antibody (Ab13) conjugated to eight molecules of topoisomerase I inhibitor exatecan through T1000, a novel self-immolative moiety. MTX-13 exhibited PTK7-specific cell binding, efficient internalization, and exatecan release to cause cytotoxic activity through DNA damage and apoptosis induction, and a strong bystander killing. MTX-13 displayed potent antitumor activities on cell line-derived xenograft and patient-derived xenograft models from a wide range of solid tumors, significantly outperforming h6M24-vc0101. PTK7 was shown to be an actionable target in small cell lung cancer for which MTX-13 showed complete and durable responses. With a consistent overexpression of PTK7 in squamous cell carcinomas derived from diverse anatomic sites, strong potency of MTX-13 in this group of heterogenous tumors suggested a common treatment strategy. Finally, MTX-13 inhibited tumor growth and metastasis in an orthotopic colon cancer xenograft model. MTX-13 displayed a favorable pharmacokinetic and safety profile in monkeys with the highest non-severely toxic dose (HNSTD) of ≥30 mg/kg, significantly higher than 3-5 mg/kg of HNSTD for h6M24-vc0101. The higher therapeutic index of MTX-13 bodes well for its clinical translation with the potential to expand the responding patient population beyond that of current PTK7-targeting ADCs.

Mechanistic insight into the repair of C8-linked pyrrolobenzodiazepine monomer-mediated DNA damage

Pyrrolobenzodiazepines (PBDs) are naturally occurring DNA binding compounds that possess anti-tumor and anti-bacterial activity. Chemical modifications of PBDs can result in improved DNA binding, sequence specificity and enhanced efficacy. More recently, synthetic PBD monomers have shown promise as payloads for antibody drug conjugates and anti-bacterial agents. The precise mechanism of action of these PBD monomers and their role in causing DNA damage remains to be elucidated. Here we characterized the damage-inducing potential of two C8-linked PBD bi-aryl monomers in Caulobacter crescentus and investigated the strategies employed by cells to repair the same. We show that these compounds cause DNA damage and efficiently kill bacteria, in a manner comparable to the extensively used DNA cross-linking agent mitomycin-C (MMC). However, in stark contrast to MMC which employs a mutagenic lesion tolerance pathway, we implicate essential functions for error-free mechanisms in repairing PBD monomer-mediated damage. We find that survival is severely compromised in cells lacking nucleotide excision repair and to a lesser extent, in cells with impaired recombination-based repair. Loss of nucleotide excision repair leads to significant increase in double-strand breaks, underscoring the critical role of this pathway in mediating repair of PBD-induced DNA lesions. Together, our study provides comprehensive insights into how mono-alkylating DNA-targeting therapeutic compounds like PBD monomers challenge cell growth, and identifies the specific mechanisms employed by the cell to counter the same.

Covalent DNA Binding Is Essential for Gram-Negative Antibacterial Activity of Broad Spectrum Pyrrolobenzodiazepines

It is urgent to find new antibiotic classes against multidrug-resistant bacteria as the rate of discovery of new classes of antibiotics has been very slow in the last 50 years. Recently, pyrrolobenzodiazepines (PBDs) with a C8-linked aliphatic-heterocycle have been identified as a new broad-spectrum antibiotic class with activity against Gram-negative bacteria. The active imine moiety of the reported lead pyrrolobenzodiazepine compounds was replaced with amide to obtain the non-DNA binding and noncytotoxic dilactam analogues to understand the structure-activity relationship further and improve the safety potential of this class. The synthesised compounds were tested against panels of multidrug-resistant Gram-positive and Gram-negative bacteria, including WHO priority pathogens. Minimum inhibitory concentrations for the dilactam analogues ranged from 4 to 32 mg/L for MDR Gram-positive bacteria, compared to 0.03 to 2 mg/L for the corresponding imine analogues. At the same time, they were found to be inactive against MDR Gram-negative bacteria, with a MIC > 32 mg/L, compared to a MIC of 0.5 to 32 mg/L for imine analogues. A molecular modelling study suggests that the lack of imine functionality also affects the interaction of PBDs with DNA gyrase. This study suggests that the presence of N10-C11 imine moiety is crucial for the broad-spectrum activity of pyrrolobenzodiazepines.

Design, Synthesis, and Bioevaluation of a Novel Hybrid Molecular Pyrrolobenzodiazepine-Anthracenecarboxyimide as a Payload for Antibody-Drug Conjugate

A novel series of hybrid molecules combining pyrrolobenzodiazepine (PBD) and anthracenecarboxyimide pharmacophores were designed, synthesized, and tested for in vitro cytotoxicity against various cancer cell lines. The most potent compound from this series, 37b3, exhibited a subnanomolar level of cytotoxicity with an IC₅₀ of 0.17-0.94 nM. 37b3 induced DNA damage and led to tumor cell cycle arrest and apoptosis. We employed 37b3 as a payload to conjugate with trastuzumab to obtain the antibody-drug conjugate (ADC) T-PBA. T-PBA maintained its mode of target and internalization ability of trastuzumab. We demonstrated that T-PBA could be degraded through the lysosomal pathway to release the payload 37b3 after internalization. T-PBA showed a powerful killing effect on Her2-positive cancer cells in vitro. Furthermore, T-PBA significantly inhibited tumor growth in gastric and ovarian cancer xenograft mouse models without overt toxicity. Collectively, these studies suggest that T-PBA represents a promising new ADC that deserves further investigation.

Structure-Activity Relationships of Antibody-Drug Conjugates: A Systematic Review of Chemistry on the Trastuzumab Scaffold

Antibody-drug conjugates (ADCs) are a rapidly growing class of cancer therapeutics that seek to overcome the low therapeutic index of conventional cytotoxic agents. However, realizing this goal has been a significant challenge. ADCs comprise several independently modifiable components, including the antibody, payload, linker, and bioconjugation method. Many approaches have been developed to improve the physical properties, potency, and selectivity of ADCs. The anti-HER-2 antibody trastuzumab, first approved in 1998, has emerged as an exceptional targeting agent for ADCs, as well as a broadly used platform for testing new technologies. The extensive work in this area enables the comparison of various linker strategies, payloads, drug-to-antibody ratios (DAR), and mode of attachment. In this review, these conjugates, ranging from the first clinically approved trastuzumab ADC, ado-trastuzumab emtansine (Kadcyla), to the latest variants are described with the goal of providing a broad overview, as well as enabling the comparison of existing and emerging conjugate technologies.

Efficacy, Tolerability, and Pharmacokinetic Studies of Antibody-Drug Conjugates Containing a Low-Potency Pyrrolobenzodiazepine Dimer

Antibody-drug conjugate (ADC) research has typically focused on the release of highly potent cytotoxic agents to achieve antitumor efficacy. However, recently approved ADCs trastuzumab deruxtecan and sacituzumab govitecan release lower-potency topoisomerase inhibitors. This has prompted interest in ADCs that release lower-potency cytotoxic drugs to potentially enhance therapeutic index and reduce unwanted toxicity. Pyrrolobenzodiazepine (PBD) dimer ADCs have been widely investigated in human clinical trials, which have focused on high-potency PBDs. In this study, we evaluated five ADCs that release the low-potency PBD dimer SG3650. The relatively low cLogD for this agent facilitated higher drug-to-antibody ratio (DAR) conjugation without the need for antibody engineering or functionalization of the drug. The rank order of potency for DAR 2 site-specific ADCs (conjugated at the C239i position) matched the order for the corresponding free drugs in vitro. Despite free drug SG3650 being inactive in vivo, the DAR 2 ADCs derived from the corresponding drug-linker SG3584 showed antitumor efficacy in solid (anti-HER2) and hematological (anti-CD22) xenograft models. Antitumor activity could be enhanced by conjugating SG3584 to trastuzumab at higher DARs of 4 and 8 and by adjusting dosing and schedule. Higher-DAR conjugates were stable and displayed good rat pharmacokinetic profiles as measured by ELISA and LC-MS/MS. A single intravenous dose of isotype control SG3584 DAR 2 ADC resulted in no mortality in rats or monkeys at doses of up to 25 and 30 mg/kg, respectively. These findings suggest that further investigations of low-potency PBD dimers in ADCs that target hematological and solid tumors are warranted.

Discovery of Novel Polyamide-Pyrrolobenzodiazepine Hybrids for Antibody-Drug Conjugates

Pyrrolobenzodiazepine (PBD) dimers are well-known highly potent antibody drug conjugate (ADC) payloads. The corresponding PBD monomers, in contrast, have received much less attention from the ADC community. We prepared several novel polyamide-linked PBD monomers and evaluated their utility as ADC payloads. The unconjugated polyamide-PBD hybrids exhibited potent antiproliferative activity (IC50 range: 10^-11-10^-8 M) against a variety of HER2-expressing cancer cell lines. Several peptide-linked variants of the lead compound were prepared and conjugated to trastuzumab to afford ADCs with drug-to-antibody (DAR) ratios ranging from 3-5. The ADCs exhibited antigen-dependent cytotoxicity in vitro and potently suppressed tumor xenograft growth in vivo in a target-dependent manner. Moreover, the ADCs were well-tolerated in both mouse and rat. This work demonstrates for the first time that PBD polyamide hybrids can serve as effective ADC payloads.

A Patent Review on FDA-Approved Antibody-Drug Conjugates, Their Linkers and Drug Payloads

Antibody-drug conjugates (ADCs) have emerged as a promising class of biologics since the first approval of Gemtuzumab ozogamicin in 2000. Compared to small molecule drugs, ADCs are structurally much more complex as they comprise of an antibody conjugated to cytotoxic payloads by specially-designed linkers. Correspondingly, the ADC patent landscape is also much more complex. This review collates and discusses the patents protecting ADCs approved by the FDA up to 31 December 2021, with particular emphasis on their linker and cytotoxin payload technologies.

Convection enhanced delivery of EGFR targeting antibody-drug conjugates Serclutamab talirine and Depatux-M in glioblastoma patient-derived xenografts

Background

EGFR targeting antibody-drug conjugates (ADCs) are highly effective against EGFR-amplified tumors, but poor distribution across the blood–brain barrier (BBB) limits their efficacy in glioblastoma (GBM) when administered systemically. We studied whether convection-enhanced delivery (CED) can be used to safely infuse ADCs into orthotopic patient-derived xenograft (PDX) models of EGFRvIII mutant GBM.

Methods

The efficacy of the EGFR-targeted ADCs depatuxizumab mafodotin (Depatux-M) and Serclutamab talirine (Ser-T) was evaluated in vitro and in vivo. CED was performed in nontumor and tumor-bearing mice. Immunostaining was used to evaluate ADC distribution, pharmacodynamic effects, and normal cell toxicity.

Results

Dose-finding studies in orthotopic GBM6 identified single infusion of 2 μg Ser-T and 60 μg Depatux-M as safe and effective associated with extended survival prolongation (>300 days and 95 days, respectively). However, with serial infusions every 21 days, four Ser-T doses controlled tumor growth but was associated with lethal toxicity approximately 7 days after the final infusion. Limiting dosing to two infusions in GBM108 provided profound median survival extension of over 200 days. In contrast, four Depatux-M CED doses were well tolerated and significantly extended survival in both GBM6 (158 days) and GBM108 (310 days). In a toxicity analysis, Ser-T resulted in a profound loss in NeuN+ cells and markedly elevated GFAP staining, while Depatux-M was associated only with modest elevation in GFAP staining.

Conclusion

CED of Depatux-M is well tolerated and results in extended survival in orthotopic GBM PDXs. In contrast, CED of Ser-T was associated with a much narrower therapeutic window.

Clinical Pharmacology of Antibody-Drug Conjugates

Antibody-drug conjugates (ADCs) are biopharmaceutical products where a monoclonal antibody is linked to a biologically active drug (a small molecule) forming a conjugate. Since the approval of first ADC (Gemtuzumab ozogamicin (trade name: Mylotarg)) for the treatment of CD33-positive acute myelogenous leukemia, several ADCs have been developed for the treatment of cancer. The goal of an ADC as a cancer agent is to release the cytotoxic drug to kill the tumor cells without harming the normal or healthy cells. With time, it is being realized that ADCS can also be used to manage or cure other diseases such as inflammatory diseases, atherosclerosis, and bacteremia and some research in this direction is ongoing. The focus of this review is on the clinical pharmacology aspects of ADC development. From the selection of an appropriate antibody to the finished product, the entire process of the development of an ADC is a difficult and challenging task. Clinical pharmacology is one of the most important tools of drug development since this tool helps in finding the optimum dose of a product, thus preserving the safety and efficacy of the product in a patient population. Unlike other small or large molecules where only one moiety and/or metabolite(s) is generally measured for the pharmacokinetic profiling, there are several moieties that need to be measured for characterizing the PK profiles of an ADC. Therefore, knowledge and understanding of clinical pharmacology of ADCs is vital for the selection of a safe and efficacious dose in a patient population.

Natural Product Based Antibody Drug Conjugates: Clinical Status as of November 9, 2020

As of early November 2020, there are 10 approved antibody drug conjugates (ADCs) plus two others that are not usually listed. In addition, there are 70 ADCs at stages from phase I to phase III and 23 that are at the preclinical stage. The warheads of all of these drugs and drug candidates have their origins in natural product structures. The sources and modifications are discussed in general and then specifically commented on in each case with either the generic name if known and/or the ADC's ID names. Interestingly, almost all warheads listed are from microbial sources though initially a number were thought to have been from plants. The latest NCT numbers from Clintrials.gov of all phase I to phase III candidates are also given. Three unusual ADCs are also discussed, two of which (an antitumor agent and one directed against autoimmune diseases) are not usually listed as ADCs, with the third being an anti-infective.

Rova-T enhances the anti-tumor activity of anti-PD1 in a murine model of small cell lung cancer with endogenous Dll3 expression

Rovalpituzumab tesirine (Rova-T) offers a targeted therapy for ~85% of SCLC patients whose tumors express DLL3, but clinical dosing is limited due to off-target toxicities. We hypothesized that a sub-efficacious dose of Rova-T combined with anti-PD1, which alone shows a clinical benefit to ~15% of SCLC patients, might elicit a novel mechanism of action and extend clinical utility. Using a pre-clinical murine SCLC tumor model that expresses Dll3 and has an intact murine immune system, we found that sub-efficacious doses of Rova-T with anti-PD1 resulted in enhanced anti-tumor activity, compared to either monotherapy. Multiplex immunohistochemistry (IHC) showed CD4 and CD8 T-cells primarily in normal tissue immediately adjacent to the tumor. Combination treatment, but not anti-PD1 alone, increased Ki67+/CD8 T-cells and Granzyme B+/CD8 in tumors by flow cytometry and IHC. Antibody depletion of T-cell populations showed CD8+ T-cells are required for in vivo anti-tumor efficacy. Whole transcriptome analysis as well as flow cytometry and IHC showed that Rova-T activates dendritic cells and increases Ccl5, Il-12, and Icam more than anti-PD1 alone. Increased tumor expression of PDL1 and MHC1 following Rova-T treatment also supports combination with anti-PD1. Mice previously treated with Rova-T + anti-PD1 withstood tumor re-challenge, demonstrating sustained anti-tumor immunity. Collectively our pre-clinical data support clinical combination of sub-efficacious Rova-T with anti-PD1 to extend the benefit of immune checkpoint inhibitors to more SCLC patients.

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Sub-efficacious Rova-T combined with anti-PD1 regresses murine SCLC tumors.

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Anti-SCLC efficacy seen with Rova-T + anti-PD1 requires CD8 T-cells.

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Rova-T + anti-PD1 recruits and activates T-cells and dendritic cells within the tumor.

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Combination of Rova-T and anti-PD1 results in prolonged anti-tumor immunity.

Pre-clinical studies of EC2629, a highly potent folate- receptor-targeted DNA crosslinking agent

Folate receptor (FR)-targeted small molecule drug conjugates (SMDCs) have shown promising results in early stage clinical trials with microtubule destabilizing agents, such as vintafolide and EC1456. In our effort to develop FR-targeted SMDCs with varying mechanisms of action, we synthesized EC2629, a folate conjugate of a DNA crosslinking agent based on a novel DNA-alkylating moiety. This agent was found to be extremely potent with an in vitro IC50 ~ 100× lower than folate SMDCs constructed with various microtubule inhibitors. EC2629 treatment of nude mice bearing FR-positive KB human xenografts led to cures in 100% of the test animals with very low dose levels (300 nmol/kg) following a convenient once a week schedule. The observed activity was not accompanied by any noticeable weight loss (up to 20 weeks post end of dosing). Complete responses were also observed against FR-positive paclitaxel (KB-PR) and cisplatin (KB-CR) resistant models. When evaluated against FR-positive patient derived xenograft (PDX) models of ovarian (ST070), endometrial (ST040) and triple negative breast cancers (ST502, ST738), EC2629 showed significantly greater anti-tumor activity compared to their corresponding standard of care treatments. Taken together, these studies thus demonstrated that EC2629, with its distinct DNA reacting mechanism, may be useful in treating FR-positive tumors, including those that are classified as drug resistant.

Antibody-drug conjugates (ADCs) delivering pyrrolobenzodiazepine (PBD) dimers for cancer therapy

INTRODUCTION

The rationally designed pyrrolobenzodiazepine (PBD) dimers emerged around ten years ago as a new class of drug component for antibody-drug conjugates (ADC). They produce highly cytotoxic DNA cross-links, exploiting a completely different cellular target to the auristatin and maytansinoid tubulin inhibitor classes and a different mode of DNA damage to other DNA interacting warheads such as calicheamicin.

AREAS COVERED

The properties which make the PBD dimers suitable warheads for ADCs, and the development of the two main payload structures talirine and tesirine, are discussed. The clinical experience with the twenty PBD dimer-containing ADCs to enter the clinic is reviewed, with a focus on vadastuximab talirine and rovalpituzumab tesirine, both of which were discontinued following pivotal studies, and loncastuximab tesirine and camidanlumab tesirine which are progressing towards approval.

EXPERT OPINION

Reviewing the clinical efficacy and safety data from almost forty clinical trials of PBD dimer-containing ADCs highlights the complexities and challenges of ADC early clinical development. It enables some conclusions to be made about reasons for failure and suggests strategies to optimise the future clinical development of this promising class of ADCs in a rapidly expanding field.

Therapeutic Monoclonal Antibodies and Antibody Products: Current Practices and Development in Multiple Myeloma

Immunotherapy is the latest innovation for the treatment of multiple myeloma (MM). Monoclonal antibodies (mAbs) entered the clinical practice and are under evaluation in clinical trials. MAbs can target highly selective and specific antigens on the cell surface of MM cells causing cell death (CD38 and CS1), convey specific cytotoxic drugs (antibody-drug conjugates), remove the breaks of the immune system (programmed death 1 (PD-1) and PD-ligand 1/2 (L1/L2) axis), or boost it against myeloma cells (bi-specific mAbs and T cell engagers). Two mAbs have been approved for the treatment of MM: the anti-CD38 daratumumab for newly-diagnosed and relapsed/refractory patients and the anti-CS1 elotuzumab in the relapse setting. These compounds are under investigation in clinical trials to explore their synergy with other anti-MM regimens, both in the front-line and relapse settings. Other antibodies targeting various antigens are under evaluation. B cell maturation antigens (BCMAs), selectively expressed on plasma cells, emerged as a promising target and several compounds targeting it have been developed. Encouraging results have been reported with antibody drug conjugates (e.g., GSK2857916) and bispecific T cell engagers (BiTEs^®), including AMG420, which re-directs T cell-mediated cytotoxicity against MM cells. Here, we present an overview on mAbs currently approved for the treatment of MM and promising compounds under investigation.

Antibody-drug conjugates in clinical trials for lymphoid malignancies and multiple myeloma

Antibody-drug conjugates (ADC) represent a distinct family of chemoimmunotherapy agents. ADCs are composed of monoclonal antibodies conjugated to cytotoxic payloads via specialized chemical linkers. ADCs therefore combine the immune therapy with targeted chemotherapy. Due to the distinct biomarkers associated with lymphocytes and plasma cells, ADCs have emerged as a promising treatment option for lymphoid malignancies and multiple myeloma. Several ADCs have been approved for clinical applications: brentuximab vedotin, inotuzumab ozogamicin, moxetumomab pasudotox, and polatuzumab vedotin. More novel ADCs are under clinical development. In this article, we summarized the general principles for ADC design, and updated novel ADCs under various stages of clinical trials for lymphoid malignancies and multiple myeloma.