This is the first clinical trial to evaluate once-daily dosing of veliparib in combination with a chemotherapeutic agent. Given its short half-life (5.0 ± 1.5 hours), veliparib has been administered twice daily in all other combination trials. We evaluated once-daily dosing based on PD data from our phase 0 trial, which showed greater than 48% inhibition of PARP activity in tumor biopsies 24 hours after a single 50 mg dose (8
), and because of better patient compliance with once-daily dosing (24
). Enhanced myelotoxicity has been observed with PARP inhibitor combination regimens with cytotoxic chemotherapy, requiring dose reduction of the chemotherapy (26
). This has raised questions about the relative contribution of the addition of PARP inhibitors to combination regimens and whether administration of full doses of chemotherapy alone would provide similar benefit. Therefore, we designed a regimen that would be well tolerated and have low levels of toxicity and good patient compliance, a potentially better therapeutic ratio, and we were able to safely escalate the PARP inhibitor dose with metronomic cyclophosphamide dosing. The regimen was well tolerated, and the MTD was established at 60 mg veliparib with 50 mg cyclophosphamide administered daily in 21-day cycles.
Six of seven partial responses were observed in patients with known BRCA
mutations, and an additional three patients with BRCA mutations had prolonged disease stabilization. Evidence of clinical benefit and PARP inhibition was observed across dose levels, suggesting that even at lower doses, veliparib produced sufficient inhibition of PARP activity to provide benefit in BRCA
-positive patients receiving DNA-damaging chemotherapy. Two of the partial responses, and two of the prolonged stable diseases, occurred in patients who had received prior cyclophosphamide. One patient with metastatic ovarian cancer and a known BRCA
mutation had complete disappearance of radiologic evidence of disease, even though tumor markers remained elevated. To our knowledge, this is the first report of the disappearance of radiologic evidence of disease in a patient treated with veliparib. Due to the unknown BRCA
status of the remaining patients, no comparative analysis with wild-type BRCA
patients can be performed. However, these preliminary data support the hypothesis that tumors with DNA repair defects may be sensitive to PARP inhibitors. Interestingly, a patient with Muir-Torre syndrome, an autosomal dominant genetic disorder likely caused by microsatellite instability (29
), and urothelial malignancy had prolonged disease stabilization for 17 cycles, further supporting the potential therapeutic role of PARP inhibitors in the treatment of tumors with DNA repair defects.
We observed a statistically significant inhibition of PARP activity in PBMCs and tumor biopsy samples across dose levels. The degree and duration of PARP inhibition in tumor required for clinical benefit has not been established. A comparative study of the effects of treatment with other PARP inhibitors, MK-4827, olaparib, and PF-01367338, on PBMCs has recently been reported, and decreases in PAR levels ranged from 65% to 92% (30
). The PAR levels at baseline and the degree of inhibition in PBMCs is variable as evidenced by our observations in this study. In the small subset of patients who underwent tumor biopsies and PBMC sampling on the same day (patients 20, 22, and 34), there appeared to be concordance in the inhibition of PAR in both sample sets. Although there is inherent variability in the baseline levels of PAR in PBMCs and the degree of PARP inhibition for a given dose level as shown in , overall PAR levels were significantly decreased in PBMCs across all dose levels.
We did not observe consistent increases in γH2AX, a sensitive marker of DNA damage (20
), in PBMCs. As previously reported in our phase 0 study of veliparib (8
), PARP activity is more easily inhibited in tumor cells than PBMCs; therefore, we evaluated the number of CTCs and the presence of γH2AX as a marker of drug effect in tumor. Though we observed increases in the fraction of CTCs positive for γH2AX after treatment, definitive conclusions cannot be made due to the few patients who underwent CTC sampling (as this was added later to the study) and low number of CTCs recovered per sample. We did not observe increases in γH2AX in PBMCs in the majority of patients evaluated, but did observe increases in γH2AX in CTCs, which is consistent with the minimal myelosuppression and promising antitumor activity observed with this regimen.
We were concerned about the potential for co-administration of cyclophosphamide to increase PARP inhibitor metabolism, because cyclophosphamide can induce CYP3A4 expression in human hepatocytes and liver slices (32
). However, no effect of metronomic cyclophosphamide on veliparib PK was observed between the first and last days of treatment (days 1 and 7, 14, or 21). These results are consistent with our previous report where 31% to 115% of the veliparib dose was recovered in urine as unchanged parent drug (8
Because of the encouraging activity and tolerability of this combination in patients with DNA repair deficiencies, the activity of metronomic cyclophosphamide alone and in combination with veliparib is being compared in a multicenter, randomized phase II study in patients with advanced ovarian cancer and BRCA mutations, high-grade serous ovarian cancers, triple-negative breast cancers, and low-grade lymphomas [ClinicalTrials.gov Identifier: NCT01306032]. The trial includes a detailed genetic analysis of underlying DNA repair defects for the ovarian cancer cohort. Veliparib as a single agent is being evaluated in a separate phase I clinical trial in patients with cancer carrying the BRCA mutation [ClinicalTrials.gov Identifier: NCT00892736]. Data from the single-agent trial were not available at the time of designing the phase II trial of the combination to add single-agent veliparib as a comparator arm.
The phase II trial, as currently designed, should help define the contribution of PARP inhibition to the activity of this combination, and will begin to establish whether a true increase in the therapeutic ratio of a cytotoxic is possible with the use of PARP inhibitors in the clinic.