A phase 1b study of chemoimmunotherapy with pegylated liposomal doxorubicin and pembrolizumab in estrogen receptor-positive, endocrine-resistant breast cancer.
Authors: Gabizon Alberto A. , Cherny Nathan , Isacson Rut , Abu Remilah Areen , Gabizon Alberto , Shmeeda Hilary , Rosengarten Ora 
Published: June 2021

1049 Background: This is a single center phase 1b study of a regimen of pembrolizumab (PBZ) and pegylated liposomal doxorubicin (PLD) in endocrine-resistant breast cancer. PLD was chosen as chemotherapy component because it is mildly myelosuppressive and non-immunosuppressive and contains doxorubicin, a strong immunogenic cell death inducer. Methods: Patients with estrogen receptor positive, HER2 negative, metastatic breast cancer, whose disease progressed on hormonal and biological therapy and up to 2 chemotherapy lines were eligible for enrollment. PLD, 30 mg/m2, and PBZ, 200 mg flat dose, were infused on day 1 of every 3-week cycles. The main study objectives were safe dose clearance, characterization of dose-limiting toxicities (DLT), tumor response, and pharmacokinetic analysis of PLD and PBZ during the first 3 cycles of treatment in a 1st cohort of 6 patients and a 2nd confirmatory cohort of 6-9 patients. Patients with partial response (PR) or stable disease (SD) continued on the extended phase of the study consisting of 9 additional cycles during which further safety information was collected. All patients were followed-up for survival. Results: 12 patients were recruited (median age 61 y, range 45-91). 9 patients had received prior doxorubicin treatment. 82 treatments have been administered (median: 7, range 2-13). Overall, treatment was well tolerated. DLT including infusion reactions, grade ≥2 myelosuppression, hair loss and mucocutaneous toxicity were not observed in the first 3 cycles. Subsequently, skin toxicity (grade 2-3 palmar-plantar erythema) was observed forcing treatment delays of 1-2 weeks. Except for 2 cases of subclinical hypothyroidism, there were no other apparent PBZ-related side-effects. There was no evidence of cardiac toxicity. There were 2 early deaths (days 25 and 45) probably related to disease progression. Upon reevaluation on week 9, we observed: 2 patients with PD, 4 with SD, 2 with PR (15+ and 5+ mth), 1 with no measurable disease, and 1 early to evaluate. Three out of 5 patients responded well to post-study chemotherapy with durable improvement or stabilization (range, 5 to 11+ mth). Median follow-up is 14 mth. Median survival has not been reached with 4 deaths and a longest survivor of 19+ mth. Median progression-free survival is 6.0 mth. The clearance of PLD was slow with high Cmax, long T½ and small Vd. There was a significant increase in the AUC of PLD between the 1st and 3rd cycle (median: 2,649 vs 3,422 mg*h/l, p = 0.039). Analysis of PBZ plasma levels is ongoing. Conclusions: The combination of PLD and PBZ is well tolerated and feasible for extended treatment. Dose interval of PLD should be lengthened to 4 weeks after 2-3 cycles to prevent skin toxicity. The late appearance of skin toxicity is probably related to a delay in PLD clearance after 2 treatment cycles with PLD and PBZ. Clinical trial information: NCT03591276 .