The development of
antibody-drug conjugates (ADC), a promising class of
anticancer agents, has traditionally relied on the use of
antibodies capable of selective internalization in
tumor cells. We have recently shown that also noninternalizing
antibodies, coupled to cytotoxic drugs by means of
disulfide linkers that can be cleaved in the
tumor extracellular environment, can display a potent therapeutic activity. Here, we have compared the
tumor-targeting properties, drug release rates, and therapeutic performance of two ADCs, based on the
maytansinoid DM1 thiol drug and on the
F8 antibody, directed against the alternatively spliced Extra Domain A (EDA) domain of
fibronectin. The antibody was used in
IgG or in small immune
protein (SIP) format. In both cases, DM1 was coupled to unpaired
cysteine residues, resulting in a
drug-antibody ratio of 2. In biodistribution studies,
SIP(F8)-SS-DM1 accumulated in the
tumor and cleared from circulation more rapidly than IgG(F8)-SS-DM1. However, the ADC based on the
IgG format exhibited a higher
tumor uptake at later time points (e.g., 33%IA/g against 8%IA/g at 24 hours after
intravenous administration). In mouse plasma, surprisingly, the ADC products in
IgG format were substantially more stable compared with the SIP format (half-lives >48 hours and <3 hours at 37°C, respectively), revealing a novel mechanism for the control of
disulfide-based drug release rates.
Therapy experiments in immunocompetent mice bearing murine F9
tumors revealed that
SIP(F8)-SS-DM1 was more efficacious than IgG(F8)-SS-DM1 when the two products were compared either in an equimolar basis or at equal milligram doses.