Pharmacokinetics of Alemtuzumab Used as Conditioning Agent in Reduced Intensity Transplantation for Pediatric Non-Malignant Disorders.

Background: Alemtuzumab (Campath-1H), a depleting humanized monoclonal antibody against CD52 has been successfully used for immunosuppression in reduced intensity transplantation (RIT) for non-malignant disorders in children (Shenoy et al, BMT 2005). The conditioning regimen used in this study included alemtuzumab on days -22 to -19, fludarabine 150mg/m2/day on days -8 to -4, and melphalan 140 mg/m2 on day -3. Doses were reduced for children<10kg (fludarabine 1mg/kg/day, melphalan 4.7mg/kg). Stem cell sources included related or unrelated matched or single locus mismatched bone marrow, cord blood, or peripheral blood.

Aims: Pharmacokinetics and clearance of alemtuzumab following conditioning for RIT as above were determined in serial fashion to correlate with transplant related outcomes. All recipients were children undergoing RIT for non-malignant disorders.

Methods: Alemtuzumab levels were measured serially in 29 patients who received the above conditioning regimen and RIT for hemoglobinopathies, bone marrow failure syndromes, metabolic disorders, or immune deficiencies/dysregulation. Recipients ≥10 kg received a total dose of 48 mg of alemtuzumab; those <10 kg received 33 mg. Serum levels were determined using the Campath antigen CD52 fusion protein (TFYO. CAMG2A.A6.1F2) by ELISA at BioAnaLab, UK on batched frozen samples on day -19 (end of infusion), day 0, day +30, and day +100 post-transplant. Outcomes are reported by patient size and include graft-versus-host-disease (GVHD), infection, graft rejection (GR), and survival. Kinetics of lymphocyte function and subpopulation recovery were serially tracked for 1 year post-transplant.

Results: Of 28 patients, 1 died of adenovirus infection prior to engraftment. Donor engraftment was present in 92.5% (25 of 27 patients). Of these, 11 patients were <10 kg (total alemtuzumab dose 33 mg) and 17 were ≥10 kg (total dose 48 mg). Table 1 shows median and range of doses administered per kg as well as pharmacokinetics and transplant outcomes classified by patient size. Alemtuzumab was detected on day +100 in 4 recipients. Overall mortality was 3.5%; Graft rejection occurred in 10.7% - all patients were in the >25 kg weight category; 36% developed aGVHD (grade 1 –4) (higher prevalence in larger recipients). Acute toxicities were predominantly bacterial and viral infections within the first 100 days after transplant and diminished significantly following immune reconstitution. Other toxicities included reversible autoimmune hemolytic anemia in 5 patients.

Conclusions: Alemtuzumab doses between 2.1 and 7.4mg/kg were well tolerated in the transplant setting and resulted in stable engraftment and low incidence of mortality and GVHD. Alemtuzumab serum levels were variable even among children with similar weights and presumably depend on the recipient CD52+ cell load. Even when administered >2 weeks prior to transplant, alemtuzumab levels were detectable as late as 30–100 days post-transplant. There was a trend towards increased graft rejection with lower alemtuzumab dosing per kg body weight suggesting that a higher dose in larger patients could benefit transplant outcomes.