PLX038: A Long-Acting Topoisomerase I Inhibitor With Robust Antitumor Activity in ATM-Deficient Tumors and Potent Synergy With PARP Inhibitors

Could a Long-Acting Prodrug of SN-38 be Efficacious in Sacituzumab Govitecan-Resistant Tumors?

We previously proposed that sacituzumab govitecan (SG, Trodelvy®) likely acts as a simple prodrug of systemic SN-38 as well as an antibody drug conjugate (ADC). In the present commentary, we assess whether a long-acting SN-38 prodrug, such as PLX038, might be efficacious in SG-resistant patients. We first describe possible mechanisms of action of SG, with new insights on pharmacokinetics and TROP2 receptor occupancy. We argue that SG is not an optimal conventional ADC and that the amount of systemic SN-38 spontaneously hydrolyzed from the ADC is so high it must have activity. Then, we describe the concept of time-over-target as related to the pharmacology of SG and PLX038 as SN-38 prodrugs. To be clear, we are not in any way suggesting that PLX038 or any SN-38 prodrug is superior to SG as an anticancer agent. Clearly, SG has the benefit over antigen-independent SN-38 prodrugs in that it targets cells with the TROP2 receptor. However, we surmise that PLX038 should be a more efficacious and less toxic prodrug of systemic SN-38 than SG. Finally, we suggest possible mechanisms of SG resistance and how PLX038 might perform in the context of each. Taken together, we argue that-contrary to many opinions-SG does not exclusively act as a conventional ADC, and propose that PLX038 may be efficacious in some settings of SG-resistance.

A Very Long-acting Exatecan and Its Synergism with DNA Damage Response Inhibitors

Exatecan (Exa) is a very potent inhibitor of topoisomerase I and anticancer agent. It has been intensively studied as a single agent, a large macromolecular conjugate and as the payload component of antigen-dependent antibody-drug conjugates. The current work describes an antigen-independent conjugate of Exa with polyethylene glycol (PEG) that slowly releases free Exa. Exa was conjugated to a 4-arm 40 kDa PEG through a β-eliminative cleavable linker. Pharmacokinetic studies in mice showed that the conjugate has an apparent circulating half-life of 12 hours, which reflects a composite of both the rate of renal elimination (half-life ∼18 hours) and release of Exa (half-life ∼40 hours). Remarkably, a single low dose of 10 μmol/kg PEG-Exa-only approximately 0.2 μmol/mouse-caused complete suppression of tumor growth of BRCA1-deficient MX-1 xenografts lasting over 40 days. A single low dose of 2.5 μmol/kg PEG-Exa administered with low but efficacious doses of the PARP inhibitor talazoparib showed strong synergy and caused significant tumor regression. Furthermore, the same low, single dose of PEG-Exa administered with the ATR inhibitor VX970 at doses of the DNA damage response inhibitor that do not affect tumor growth show high tumor regression, strong synergy, and synthetic lethality.

Significance

A circulating conjugate that slowly releases Exa is described. It is efficacious after a single dose and synergistic with ATR and PARP inhibitors.