Antibody-Drug Conjugates: Simple Idea, Complicated Matter

ntibody-drug conjugates (ADCs) are a class of pharmaceuticals that consist of small molecule drugs (also known as payloads) covalently attached to an antibody via a linker (1,2). While the majority of these products are in oncology, occasionally they are used in non-oncology indications, such as in rheumatoid arthritis (3) and infectious diseases (4). This review will discuss the use of the current generation of ADCs in oncology (i.e., ADCs that contain cytotoxic payloads) and excludes any discussion on conjugated products containing toxins, such as moxetumomab pasudotox (5). Additional information is available in ICH S9 Guidance and ICH S9 Questions and Answers for nonclinical development of ADCs in oncology (6,7) and in ICH S6 Guidance for nonclinical development of biotechnology-derived products (8).

While the ADC technology has evolved, many ADCs are designed using the same linker-payload, such as vcMMAE, mcMMAF, SMCC-DM1, and vaPBD (11,12,13). This may be partially due to the availability of technology that makes it easier to generate a new ADC and the potential concern that any new technology may be associated with uncertainties and undesired outcomes. In addition, using the same data for multiple regulatory submissions makes it less costly and faster to submit a new IND (Investigational New Drug application) or a marketing application by eliminating conduct of certain nonclinical studies needed in support of regulatory submissions.

Challenges in ADC Development

A desired outcome is to deliver the ADCs to tumor cells and spare healthy tissues of toxicities associated with the payloads, but the payloads will eventually be released and re-distribute. The product label of FDA-approved ADCs and additional data analysis for the current generation of ADCs (11,12,18) indicate that dose-limiting toxicities (DLTs) in animals and patients are mainly related to the payload. These include but are not limited to toxicities at distant sites, such as hepatotoxicity observed with Kadcyla, Adcetris, and Polivy, and ocular toxicities associated with Blenrep. The maximum tolerated doses (MTDs) in patients have been dependent on the payload and can be generally predicted for ADCs using the same linker-payload, drug-to-antibody ratio (DAR), and frequency of administration. The human MTDs were in the range of 1.8 to 2.4 mg/kg with vcMMAE-conjugated ADCs that had a DAR of 4 and were given every three weeks as an intravenous infusion (11). When the payload is associated with immune activation and pro-inflammatory responses, high inter-subject variability may be encountered, leading to a wider range of human MTDs for comparable ADCs and dosing regimen.

In 2019, we reported the difficulties pharmaceutical companies were facing when determining the human MTDs in clinical trials of pyrrolobenzodiazepine (PBD)-ADCs when 15 separate INDs were examined (12). Our analysis indicated that PBD caused multi-organ inflammatory responses. Immune-mediated events can have delayed onset and affect various organs. In addition, they may be prone to inter-subject variability and may vary depending on the genetics, previous antigen exposures, and previous anticancer treatments. The many factors contributing to the onset, nature, and severity of immune-mediated findings can make it difficult to select a dose that will be tolerated across indications and patient populations, even for ADCs with the same payload and DAR, and when using the same schedule of administration.

Another example of an immune-activating payload may be the auristatin MMAF. The dataset evaluated by the FDA in 2015 (11) included only two MMAF-ADCs; thus a thorough evaluation of toxicities was not possible. With more data becoming available, signals of pro-inflammatory responses are being detected with MMAF-ADCs and attributed to MMAF. Blenrep (belantamab mafodotin) is an MMAF-containing ADC recently approved for the treatment of patients with multiple myeloma. Based on toxicology data (FDA multi-disciplinary review for Blenrep, 2020), treatment of animals with belantamab mafodotin or the unconjugated payload resulted in pro-inflammatory responses as indicated by changes in hematology parameters and histopathology observations of multi-organ inflammation.

One of the notable findings with Blenrep described in the product label is ocular toxicity (belantamab mafodotin). Ocular toxicity was also noted in animals treated with both the payload or belantamab mafodotin, and while its cause is not entirely understood, we speculate that MMAF-related inflammatory responses may have contributed to the findings in animals. Although ocular toxicities have been previously described in patients treated with ADCs (19), these findings appear to be more evident and generally of higher severity in MMAF-ADCs and maytansinoid DM4-conjugated ADCs, with the following terms used in describing the events in patients: blurred vision, dry eye, keratopathy, microcystic keratopathy, keratitis, iridocyclitis, corneal epitheliopathy, eye pain, conjunctival hemorrhage, corneal deposits, and photophobia. It has been suggested that the difference between ocular toxicities associated with MMAF-ADCs and MMAE-ADCs may be related to the charged (for mcMMAF) versus uncharged (for vcMMAE) metabolites of auristatins and thus differences in their cell permeability (18,19,20,21). But the discussions do not explain the mechanism of ocular findings or why the eyes are more sensitive than other organs. Additional studies will be needed to better understand the cause of the ocular toxicities.

In a cross-biologic license application (BLA) comparison of FDA-approved MMAF-ADCs and MMAE-ADCs, we noted more pronounced ocular findings in animals treated with belantamab mafodotin compared to the three approved MMAE-ADCs brentuximab vedotin, polatuzumab vedotin, and enfortumab vedotin (FDA Pharmacology Review for Adcetris, 2011; FDA Pharmacology Review for Polivy, 2019; FDA multi-disciplinary review for Padcev, 2019; FDA multi-disciplinary review for Blenrep, 2020). The same pattern is noted in patients, based on the information available in the product labels.

Circulating ADCs with non-specific distribution may result in early onset of toxicities. This is based on the observation that payload-related toxicities were more evident in rodent studies where no binding to the target occurred; toxicities were less prominent or delayed in monkeys where the ADCs bound to their targets (11). This was also seen in the review of nonclinical data for belantamab mafodotin (FDA multi-disciplinary review for Blenrep) that noted more pronounced ocular toxicities of the ADC in rodents (no target binding) compared to the studies of the ADC in monkeys (binding occurs). These observations suggest that high levels of target expression may delay the emergence of payload-related toxicities.

Conclusion

Accumulated clinical and non-clinical safety data from specific linker-payload platforms have resulted in more efficient nonclinical product development by reducing the number of animal studies needed to characterize payload-related toxicities. This accumulated knowledge can also lead to more efficient dose escalation trial designs by eliminating subtherapeutic dose levels of the ADCs when human MTDs have been reported to be in a tight range regardless of the antibody, as was the case for vcMMAE-ADCs. This data can also be used to improve safety of dose escalation by urging caution in escalating beyond a set point based on experience with related products.

Payload-related toxicities continue to create challenges in the development of ADCs as described above. Despite advances in product stability to reduce deconjugation of payloads in the plasma and the selection of better targets (e.g., tumor antigens with minimal expression in healthy tissues), the payloads are eventually released and redistributed, and at times can result in severe toxicities that may halt or delay product development. The challenges associated with the current generation of ADCs have led to the birth of novel ideas such as non-toxic payloads that could be converted to an active drug with additional triggers. One such design is described in a recent article for photo-activatable ADCs (22).

New platforms can be complicated, and FDA remains ready to engage with sponsors early in a product’s development to advance safe and effective treatments for patients with cancer.