The Use of Checkpoint Inhibitors before and after Allogeneic Stem Cell Transplantion: A Double-Edged Sword

Introduction: Allogenic stem cell transplantation (allo-SCT) is a potentially curative option for hematological malignancies. Checkpoint inhibitors (CPI) have been successful in achieving remission for patients that relapse after allo-SCT. CPI can help relapsed/refractory (RR) patients to respond and bridge towards allo-SCT after achieving remission. Check point inhibition after allo-SCT carries an increased benefit of graft vs malignancy effect (GvL) but it may exaggerate the risk of immune system related toxicity such as graft versus host disease (GvHD). Methods: To assess the safety and efficacy of CPI use in conjunction with allo-SCT, after a comprehensive literature search, we included data (n=283) from a total of twenty-four studies (11 original manuscripts, 13 case reports or case series) and analysed the results. Results: Most common indication for CPI use was Hodgkin lymphoma (n=182). CPIs used in various studies included CTLA-4 inhibitors (ipilimumab, n=93) and PD-1 inhibitors (nivolumab, n=167 and pembrolizumab, n= 27). In patients who were exposed to CPI before allo-SCT (n=107), 56% patients developed acute (a) GvHD and 29% patients developed chronic (c) GvHD. The overall mortality risk (11/107) associated with GvHD was 11%. Interval between last dose of CPI and allo-SCT ranged from 28-62 days. Median cycles of CPI therapy ranged from 4-9 cycles. The overall response rate (ORR) was observed (42/62) to be 68% patients with complete remission (CR) in 47% patients and partial remission (PR) in 21% patients. Most common adverse events reported were non-infectious febrile syndrome (12%), infections (5%), hepatic sinusoidal obstruction syndrome (4%) and encephalitis (3%). In patients (n=150) who received CPI after allo-SCT for treatment of disease relapse, 13% patients developed aGvHD and 11% patients developed cGvHD. The overall mortality risk with GvHD was around 7% in this population. The interval between allo-SCT and first dose of CPI ranged from 12.5 months to 29 months. Nivolumab was given at doses 1 mg/kg to 3 mg/kg, weekly or two-weekly. Ipilimumab dose ranged from 0.1 mg/kg to 5 mg/kg. A combination with lenalidomide was also tried. Pembrolizumab was administered at 200 mg/kg every three weeks. An ORR of 48% (59/123) was observed with CR in 34 (28%), PR in 25 (20%) and disease stabilization in 7 (6%) patients. Complications, other than GvHD, include hematological side effects (22%), most notably neutropenia followed by respiratory and hepati