A DNA-damaging lupus autoantibody synergizes with PARP inhibitors against DNA repair-deficient tumor cells

The lupus anti-DNA autoantibody 3E10 is a compelling candidate for development as a targeted therapy for DNA repair-deficient malignancies. 3E10 has previously been shown to localize to tumors due to its attraction to DNA released by dying cancer cells, penetrate into cell nuclei, inhibit DNA repair, and kill cancer cells with defects in homology-directed repair (HDR) of DNA double-strand breaks. A more potent derivative of 3E10 with increased affinity for DNA has been developed (referred to here as 3E10EN), and identification of optimal combination therapies with 3E10EN is needed to facilitate planning for upcoming clinical trials. In the present study, we found that 3E10EN increases the activity of the DNA repair enzyme poly (ADP-ribose) polymerase (PARP) in HDR-deficient cells and hypothesized that combination treatment with 3E10EN and PARP inhibitors (PARPi) would yield synergistic effects on HDR-deficient cancer cell survival.

PARP content and activity in HDR-deficient and proficient cells prior to and following treatment with 3E10EN was evaluated. 3E10EN did not impact PARP protein content but yielded a significant increase in pADPr signal in HDR-deficient cells, which suggests a compensatory increase in PARP activity in response to DNA damage accumulation in HDR-deficient cells. Combinations of 3E10EN and the PARPi olaparib were tested on a panel of HDR-deficient cells, and a matched pair of BRCA2-deficient and proficient DLD1 cells. Olaparib inhibited the increase in pADPr caused by 3E10EN, and colony formation assays analyzed by the Chou-Talalay method confirmed that 3E10EN and olaparib synergized against HDR-deficient cancer cells. Conversely, HDR-proficient cells were resistant to 3E10EN and olaparib combination treatment.

The original 3E10 is a murine antibody isolated from a lupus mouse model, and in preparation for its further development as a new drug we have recently designed Deoxymab 1 (DX1), a humanized version of 3E10EN. DX1 exhibits improved activity relative to the 3E10EN prototype, and when tested on a panel of HDR-deficient and proficient cells, DX1 and olaparib exhibited synergistic effects similar to that observed with the 3E10EN prototype.

In conclusion, we have found that both the prototype 3E10EN and humanized DX1 synergize with PARPi against HDR-deficient tumor cells. These findings provide the rationale for further studies to determine the potential for this approach to be translated into a clinically relevant therapeutic strategy.