Retrospective data from the PAOLA-one trial shows a significant reduction in benefit from subsequent platinum-based therapy for patients who progressed while on a PARP inhibitor. Objective response rates dropped from 50-60% (placebo group) to 30% (PARP group).

Real-world data shows that in platinum-sensitive ovarian cancer patients who have progressed on PARP inhibitors, subsequent platinum-based chemotherapy has a surprisingly low response rate of only 20%. This quantifies a significant opportunity for highly active ADCs to potentially replace platinum in this growing patient population.

The widespread use of PARP inhibitors has altered tumor biology in platinum-sensitive ovarian cancer. A recent meta-analysis of heavily pretreated patients, 97% of whom had prior PARP inhibitor exposure, revealed an objective response rate to subsequent therapy of only 17%—far lower than historical expectations, highlighting a critical unmet clinical need.

The modest benefit of PARP inhibitors in metastatic breast cancer, compared to ovarian cancer, is likely due to resistance induced by prior exposure to DNA-damaging agents like anthracyclines. This explains the clinical rationale for moving PARP inhibitors to earlier treatment settings, such as neoadjuvant or adjuvant therapy, before resistance develops.

The risk of developing myeloid neoplasms from PARP inhibitors in the frontline ovarian cancer setting is very low, around 1%. However, it is critical to adhere to the recommended 2-3 year treatment duration and then stop the therapy to avoid unnecessary long-term risk.

While the risk of secondary leukemia (MDS/AML) from PARP inhibitors is low (0.5-1.5%) in the frontline setting, it escalates dramatically to 8-11% in heavily pretreated patients or those receiving therapy for more than two years, highlighting a significant long-term safety concern.

Giving adjuvant olaparib to BRCA-mutated patients who have already achieved a pathologic complete response (pCR) from neoadjuvant platinum-based chemotherapy is discouraged. Their prognosis is already excellent, so adding a PARP inhibitor offers little potential benefit while exposing them to unnecessary risks of toxicity, such as MDS/AML.

Despite theoretical differences in potency or PARP1 specificity, all approved PARP inhibitors demonstrate comparable clinical toxicity profiles. Oncologists should counsel patients on a consistent class effect of myelosuppression, primarily grade 3 anemia requiring transfusion in about 25-33% of patients, regardless of the specific agent.

Prolonged PARP inhibitor use, especially beyond two years in BRCA-mutated patients, significantly increases the risk of secondary malignancies like MDS/AML. The baseline risk in this population is already elevated, and PARP inhibitors exacerbate it.