A Phase II Multicenter Trial of HSV-TK Engineered Donor Lymphocytes after Haplo-Identical HSCT: Early Immune Reconstitution and Abrogation of Gvhd.

Haplo-identical stem cell transplantation (haplo-SCT) is a therapeutic option for pts with high risk hematologic malignancies lacking an HLA matched donor. The full exploitation of haplo-SCT is limited by delayed immune reconstitution and prolonged risk of life-threatening infections. We previously identified in an academic haplo-SCT trial, effective doses of donor lymphocytes genetically engineered to express a suicide gene (the herpes simplex thymidine kinase (TK-DLI) as an effective tool for preventing disease relapse and promoting immune reconstitution. In case of GvHD, TK-DLI could be selectively eliminated by ganciclovir. A phase II multicenter trial (MM TK007), aimed at verifying the therapeutic activity of early add-backs of TK+ cells after haplo-SCT in inducing a stable immune reconstitution, started in 2002. 20 pts were recruited and transplanted for AML (16, 8 of whom post MDS), ALL (2), advanced NHL and HD (2); disease status at SCT was CR1(5), CR2(7), refractory(8). Median time from diagnosis to SCT was 394 d (95-3752), median age was 52 (17–62). A median of 12.3x10⁶/kg (7.3–15.5) CD34+ selected cells (Clinimacs) and 1.05x10⁴/kg (0.8–1.4) CD3+ cells were infused after a myeloablative conditioning regimen. GvHD prophylaxis was never administered post-SCT. 18/20 pts engrafted with a median time of 13 d (8–21), for ANC >1.0x10⁹/l and 13 d (11–24) for plt >50x10⁹/l. No immune reconstitution and no GvHD were observed in absence of TK-DLI. The statistical end-point of the study was to achieve immune reconstitution in 7/18 treated pts. So far, 10 pts received TK-DLI at a median dose of 10⁷/kg with 1st infusion at d +42. Although this study will continue up to 18 treated pts, the primary end-point has been already achieved since 7/9 evaluable pts obtained CD3+ >100/μl at a median time of 76 d (61–99) from SCT and 21 d (14–31) from TK-DLI. Transduced cells were documented ex vivo in all pts; median counts at peak of immune reconstitution were 483 CD3+/μl, 304 CD8+/μl, 130 CD4+/μl. Prompt immune reconstitution resulted in complete control of viral infections and virtually eliminated late infections frequently reported after haplo-SCT. In 1 pt a grade II biopsy-proven acute GvHD, involving skin and liver was observed and completely abrogated by ganciclovir (10 mg/kg/day) in the absence of immunosuppressive drugs. One pt relapsed at d 300; overall, 6 pts are alive in CR at d +423, +363, +317, +255, +109, +104. The preliminary results of this multicenter phase II study confirm that TK-DLI is an effective tool for promoting immune reconstitution and protecting pts from infectious mortality after haplo-SCT while providing an effective and selective treatment for GvHD. The potential of this strategy in preventing disease relapse is promising and will remain the main focus of the long-term follow-up.