Early ADCs relied on drugs that were already approved for use as small molecules, such as vinblastine and doxorubicin, and suffered from lack of potency and low clinical activities. Substantially more potent drugs that were too toxic to use in an untargeted manner have been more promising as ADCs. These include auristatins, maytansines, and calicheamicins. Auristatins and maytasines both exert their cytotoxic effects by binding to tubulin, causing G2/M cell cycle arrest, and subsequently leading to apoptosis. Monomethylauristatin E, conjugated through a protease-cleavable dipeptide linker (vcMMAE), and monomethylauristatin F, conjugated directly to mAbs through maleimidocaproic acid (mcMMAF), are synthetic analogs of dolastatin 10, a natural product originally isolated from the Indian ocean sea hare Dolabella ariculara. DM1, conjugated through a disulfide or directly through the heterobifunctional succinimidyl 4-[N-maleimidomethyl]cyclohexane-1-carboxylate (SMCC) linker, and DM4, conjugated through a disulfide, are semi-synthetic analogs of maytansine, a natural product originally isolated from the Ethiopian shrub Maytenus ovatus. Calicheamicin, a DNA strand cleaving agent conjugated through an acid-sensitive hydrazone, is a semi-synthetic analog of a Micromonospora echinospora ssp. calichensis fermentation product.
Drug Potency and ADC Activity
Early ADCs used clinically approved chemotherapeutic drugs because of their ready availability, amenability to chemical manipulation and their well known toxicological properties. Much about the strengths and pitfalls of antibody-mediated drug delivery was learned from BR96-doxorubicin, an ADC directed against the LewisY tetra-saccharide antigen on human carcinomas. Doxorubicin was attached to a chimeric version of the BR96 antibody, through an acid-labile hydrazone linker. Upon binding to the antigen, the antibody was internalized and doxorubicin released within acidic endosomal and lysosomal vesicles. Treatment of tumor-bearing mice and rats with BR96-doxorubicin led to immunologically specific tumor cures, albeit at high (>100 mg/kg) doses, reflecting the low potency of the targeted drug. In a phase I clinical trial, the maximum-tolerated dose of BR96-doxorubicin was ~700 mg/m2, with dose-limiting gastrointestinal toxicities. In a subsequent phase II trial, the toxicities were attributed to normal gut expression of LewisY. In both trials, limited antitumor activity was obtained. Thus, the limitations of BR96-doxorubicin include low molar potency of the cytotoxic drug and antigen expression on sensitive normal tissue. Additionally, the hydrazone linker had a half-life of ~43 hours that was ~7-fold lower than the pharmacokinetic terminal half-life of the BR96 antibody.
Several groups hypothesized that it may be necessary to construct ADCs with drugs of much higher molar potencies than conventional chemotherapeutics to achieve therapeutically effective levels of active drug within tumors in patients. A major advance in ADC technology came with the utilization of such highly potent drugs as calicheamicins, maytansinoids, auristatins, and CC1065 analogs. This progress reflects an appreciation that the localization of antibodies to tumors is inefficient in man: typically 0.0003%-0.08% injected dose per gram of tumor.
Calicheamicins are a family of enediyne antibiotics derived from the soil microorganisim, Micromonspora echinospora ssp. calichensis. These drugs bind to the minor groove of DNA and induce double-stranded DNA breaks, resulting in cell death with >100-fold the potency of most standard chemotherapeutics. As with BR96-doxorubicin, an acid-labile hydrazone linker was used to link calicheamicin to antibody delivery vehicles, with a similar half-life for drug release from the ADC of ~48 hours. Pronounced antitumor activities have been obtained with calicheamicincontaining ADCs targeting MUC1. This calicheamicin ADC technology has also been successfully applied to antibodies recognizing a range of tumor antigens including: CD22, CD33, LewisY and oncofetal protein, 5T4. ADCs utilizing a related enediyene, calicheamicin θ11, have been used successfully to target tumors expressing ganglioside GD2 and γ-glutamyltransferase. The doses required for activity were much lower than that required for a doxorubicin ADC, reflecting the greatly increased potency of the targeted drug. The sole clinically approved ADC to date is the calicheamicin-containing conjugate, gemtuzumab ozogamicin.
Carter, P. J., & Senter, P. D. (2008). Antibody-drug conjugates for cancer therapy. The Cancer Journal, 14(3), 154-169.