ADVANCES IN DEVELOPMENT
Early improvements in endogenous protein-based therapeutics produced new, commercially successful therapeutics with desirable
properties by simply extending sequence incorporating fusion to proteins such as the constant fragment of antibodies (Fc)
or by PEGylating to increase half-life. Beyond these early approaches, considerable effort to produce ever-more elegant constructs
that combine two separate functions have been made. One promising approach, antibody drug conjugates (ADCs), involves using
a targeting antibody to known tissue selective cell-surface antigens or receptors to target conjugated toxins or cytotoxic
drugs and so enhance selectivity over normal tissue.
Successful design of effective ADCS is complex and requires linking cytotoxic drug payloads to tumor-targeting antibody constructs.
The selection of an ideal antigen target for optimal internalization and specificity for tumor tissues is critical. The design
of linkers that are stable in circulation, but cleave when internalized in tumor cells to release the cytotoxic drug, adds
to the complexity, but the other major technical hurdle has been to define how the cytotoxic payload with linker are conjugated
to the targeting antibody. The biopharmaceutical companies Seattle Genetics and Immunogen have developed robust platforms
that depend on conjugation of linkers and cytotoxic warheads to available cysteine or lysine residues, respectively, in the
tumor targeting antibody sequence.
Despite successes with ADCs, there are many examples where seemingly optimized functional components (i.e., antigen-binding
motif, linker, and drug-conjugate) do not translate into a developable therapeutic candidate. ADCs produced using conjugation
chemistries to endogenous cysteines and lysines inevitably lead to the production of multiple species of the ADC with the
drug conjugated in varying payloads of between one and nine molecules per immunoglobulin G (IgG). Furthermore, all sites of
conjugation are not equal. Some conjugations interfere with antigen-binding epitopes, thereby reducing binding affinity and/or
drug half-life (1). All too often, poor efficacy is revealed in the clinic only after significant investment in cell-based
expression systems and scale-up.
SITE-DIRECTED CONJUGATIONS
Several technologies aim to provide chemically amenable sites within a protein sequence for the posttranslational chemical
conjugation of small-molecule drugs, peptides, or other constructs to improve or add functionality. For example, sequence-specific
conjugations producing homogeneous ADCs with fixed payloads are aimed at improving tumor-cell killing and increasing therapeutic
index. Engineered ThioMabs (Roche/Genentech technology) that use natural cysteine residues that must be carefully unmasked
during production for subsequent site-specific conjugation have shown preclinical proof-of-concept (2). These approaches await
further clinical validation.
Carlos Barbas' laboratory at The Scripps Research Institute exploited the use of exposed tyrosine residues within the complimentarity
determining regions (CDRs) of IgG molecules as the basis for linking drug conjugates. CovX, acquired by Pfizer in 2008, was
founded to develop this technology. These sorts of approaches are attractive in that posttranslational chemical coupling to
a common IgG construct with resulting extended half-life represents a platform amenable to many different small-molecule or
peptide agonists.
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