Antibody-Drug Conjugates: Key Challenges in Safety Assessment
Melissa M. Schutten, DVM, PhD, Diplomate ACVP
Safety Assessment, Genentech, Inc.
South San Francisco, CA 94080 (schutten.melissa@gene.com)
Introduction
The number and types of targeted cancer therapies under development
for the treatment of human disease have greatly expanded over the last decade.
Targeted therapies, like antibody-drug conjugates, hold particular promise for
oncology patients as they are purposefully designed to minimize systemic toxicity
while delivering a highly potent, cytotoxic payload to a target tumor cell
population (1, 2). Antibody drug conjugates (ADCs), or immunoconjugates, are
hybrid molecules usually comprised of monoclonal antibodies conjugated with
potent cytotoxins, but also can consist of other molecules, such as antibody
fragments or radioisotopes (Fig. 1).
Fig. 1. Anatomy of a typical
antibody-drug conjugate.
Modes of Anti-Tumor Activity of ADCs
The antibody portion of the ADC recognizes a cell surface protein that
serves as an “address” for the therapeutic agent. Ideally, the target antigen of
interest is highly expressed on tumor cells and has low to no expression on
normal, non-neoplastic tissues. Upon ADC binding, the antigen-ADC complex is
internalized through receptor-mediated endocytosis and is transported from early
endosomes to lysosomes. Inside the lysosomal compartment, internal conditions
(e.g. acidic environment) trigger linker cleavage causing the cytotoxic payload to
be released into the cytoplasm. The type of cytotoxins used in current ADCs
varies, but the majority either bind to tubulin, resulting in microtubule disruption,
or bind to the minor groove of DNA, inducing DNA damage and strand breaks.
Cytotoxin-induced damage, regardless of the underlying mechanism described
here, results in apoptosis and preferential killing of target tumor cells (Fig. 2).
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ADCs directed against poorly internalized antigens have also been reported to
have therapeutic activity via indirect cytotoxic activity that utilizes linker cleavage
strategies extracellularly in the tumor microenvironment and the inherent
membrane permeability of specific cytotoxic drugs (3).
This bystander
mechanism allows for ADCs to be more broadly effective as it supports the use
of ADCs targeting tumors with heterogenous antigen expression and poor
internalization.
Fig. 2. Modes of
anti-tumor activity of
ADCs
Modes of Toxicity of ADCs
ADC-related toxicities are complex and can be broadly classified as those
related to specific or nonspecific uptake mechanisms and those related to
systemic release of the cytotoxic drug and overall ADC catabolism (Fig. 3). In
either scenario, ADC-related toxicities are driven by multiple factors. The
contribution of each ADC component, namely the monoclonal antibody, linker,
and cytotoxic drug, can have important effects on the overall toxicity profile,
however it is fundamental to appreciate that the activities associated with each of
these components are intertwined and will collectively modulate ADC-related
toxicities. Importantly, modulating even a single variable of the ADC can have a
measurable effect on toxicity. This concept can be illustrated by the example of
altering the total drug load on the antibody and subsequent effect on the toxicity
profile.
Standard conjugation of cytotoxic drugs to antibodies occurs through
either lysine residues or reduction of internal disulfide bonds and results in ADCs
with a heterogeneous mixture of drug-to-antibody (DAR) species (4). Increasing
drug loads on the antibody has been associated with different PK, efficacy and
safety profiles of ADCs (5, 6). For example, purified anti-HER2-MMAF ADCs, an
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Fig. 3. Modes of
toxicity of ADCs
antibody targeting the HER2 antigen conjugated to the tubulin polymerization
inhibitor MMAF, with 2, 4, or 6 MMAF moieties (corresponding to DARs of 2, 4,
and 6) caused DAR-dependent hepatic toxicities as indicated by elevated
transaminase levels following single intravenous administration in rats.
Consistent with previous reports, there was an association of increased toxicity
with increasing drug loads (DAR) on ADCs. These observations led to the
development of engineered conjugation sites (e.g. unpaired cysteine residues), in
order to have greater control of the number of drug molecules per antibody (6).
This technology, referred to as THIOMAB drug conjugates (TDCs), has different
PK and safety profiles in non-clinical species (7, 8, 9). A comparison of the PK of
MMAE ADC or TDC conjugates showed that both the catabolism and
deconjugation of TDCs were slower than the ADC in rats (7). TDCs were better
tolerated in short-term toxicology studies in rats and NHPs. As an example,
neutropenia was observed with an ADC, but not its TDC counterpart, at
equivalent MMAE doses (μg MMAE/m2), in NHPs. Neutropenia was eventually
seen at 2-fold higher doses of the TDC (7). However, additional non-hematologic
toxicity was noted in NHPs in repeat-dose studies; this toxicity is believed to
occur secondary to increased conjugate stability with the TDC format.
Summary
In summary, ADCs are being developed as novel cancer therapeutics to
offer a potentially widened therapeutic index over standard cytotoxic
chemotherapies. However, given the complexity of these molecules, there are
unique challenges involved in their safety assessment. There are many factors,
such as differences in cytotoxic drug potency, pharmacokinetic profiles and linker
stability, and target antigens, which can affect the toxicity profile. Careful
selection of individual ADC components is an important consideration for a
desirable toxicity and efficacy profile. This presentation will provide an overview
of evolving preclinical development strategies and challenges, with a particular
focus on toxicology, associated with this unique and complex drug class.
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References
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