ADCT-502

Antibody–drug conjugates (ADC) containing pyrrolobenzodiazepine (PBD) dimers are being evaluated clinically in both hematologic and solid tumors. These include ADCT-301 (camidanlumab tesirine) and ADCT-402 (loncastuximab tesirine) in pivotal phase II trials that contain the payload tesirine, which releases the PBD dimer warhead SG3199. An important consideration in future clinical development is acquired resistance. The aim was to generate and characterize PBD acquired resistant cell lines in both hematologic and solid tumor settings. Human Karpas-299 (ALCL) and NCI-N87 (gastric cancer) cells were incubated with increasing IC50 doses of ADC (targeting CD25 and HER2, respectively) or SG3199 in a pulsed manner until stable acquired resistance was established. The level of resistance achieved was approximately 3,000-fold for ADCT-301 and 3-fold for SG3199 in Karpas-299, and 8-fold for ADCT-502 and 4-fold for SG3199 in NCI-N87. Cross-resistance between ADC and SG3199, and with an alternative PBD-containing ADC or PBD dimer was observed. The acquired resistant lines produced fewer DNA interstrand cross-links, indicating an upstream mechanism of resistance. Loss of antibody binding or internalization was not observed. A human drug transporter PCR Array revealed several genes upregulated in all the resistant cell lines, including ABCG2 and ABCC2, but not ABCB1(MDR1). These findings were confirmed by RT-PCR and Western blot, and inhibitors and siRNA knockdown of ABCG2 and ABCC2 recovered drug sensitivity. These data show that acquired resistance to PBD-ADCs and SG3199 can involve specific ATP-binding cassette drug transporters. This has clinical implications as potential biomarkers of resistance and for the rational design of drug combinations.

Antibody-drug conjugates are an elegant approach to cancer treatment that couples the specificity of monoclonal antibodies to the cytotoxicity of classic chemotherapy agents, permitting, at least in theory, increased activity and reduced toxicity. In breast cancer, the early success of trastuzumab-emtansine (T-DM1) in the HER2-positive metastatic setting led to great hopes, later dashed by results in the early setting (KRISTINE trial) and in combination with pertuzumab (MARIANNE trial). Parallel to this, development of ADCs in breast cancer has suffered other setbacks, including the recent failure of other agents (MM-302) as well as the suspension of a few programs (XMT-1522, ADCT-502) with the overall effect of dampening the impetus of this concept and halting/delaying the progress of drugs associated with it, particularly when immunotherapy is at the center of so many efforts. Numerous antibody-drug conjugates remain, however, in development, and could prove successful. Critically, ADCs could permit the introduction of novel concepts such as the expansion of potent anti-HER2 therapy to HER2-low breast cancer, treatment beyond resistance to T-DM1, and synergy in combination with immune checkpoint blockade. In the early setting, the ATEMPT trial may show that T-DM1 reduces toxicity while maintaining good outcomes for lower risk HER2+ patients. ADCs based on bispecific antibodies are also in development. Finally, breakthroughs are occurring in the orphan triple-negative breast cancer subtype with agents targeting surface proteins. The recent results of Sacituzumab govitecan suggest substantial activity in heavily pre-treated patients and underscore the enduring relevance of antibody drug conjugates as a path towards better outcomes.