Targeted IV Vs Oral Busulfan in Very Young Children with Thalassemia Major Undergoing Matched Allogeneic Haematopoietic Stem Cell Transplantation
Authors: Balasubramanian Poonkuzhali , George Biju , Nagarajan Rajesh , Kulkarni Uday Prakash , Mohanan Ezhil Pavai , Edison Eunice Sindhuvi , Lakshmi Kavitha M , Samoon Yasir Jeelani , Devasia Anup , N. Fouzia , Korula Anu , Abraham Aby , Mathews Vikram , Srivastava Alok 
Published: October 2019

Abstract
Targeted intravenous (IV) Busulfan (TBu) in combination with cyclophosphamide (Cy) is widely used as a preparative regimen in children with thalassemia major undergoing matched allogeneic haematopoietic stem cell transplantation (HSCT) to reduce toxicity. At our centre, we have been using TBu/Cy regimen since 2011 for very young children (Pesaro class I/II risk status) and with matched related donors. We have compared the outcome of HSCT in this group of patients with a retrospective cohort (from 1995-2009) of age and risk status matched patients receiving fixed dose oral Bu without PK monitoring.
All patients with beta thalassaemia major undergoing HSCT with matched related donor between January, 2011 - May, 2018, receiving IV Bu (0.8mg/kg Q6H days x 4 days) / Cy (50mg/kg x 4 days) with anti-thymocyte globulin based conditioning regimen were included in this study. Busulfan levels were monitored after the 1st dose of busulfan and further doses adjusted to achieve a target range of 900-1300um*min. The Bu plasma levels achieved on day 1 and on day 3 were compared with HSCT outcome endpoints including chimerism status on D28, overall and event-free survival (OS, EFS), and graft rejection.
There were 52 children, median age of 3 years (range:1-6); 44 class I/II and 8 class III low risk. Different proprietary Bu products were used: 26 received Bucelon™ (Celon Labs, Hyderabad, India), 23 patients received Buslera™ (Biem Pharma, Ankara, Turkey), and 3 received Bufatas™ (Intas Pharma, Ahmedabad, India). The 3rd dose of Bu was increased in 35, decreased in 2 and unchanged in 15 patients. The median 1st dose Bu AUC was 625 um*min (range: 115- 2466) while the 9th dose AUC was 1105 um*min (range: 543-2656). Target Bu AUC was achieved in 40 patients (77%) as assessed by Bu AUC on Day 3 while the AUC in the remaining 12 patients (23%) was lower than 900 um*min (range: 543-872). Twenty-three of the 50 evaluable patients showed mixed chimerism (MC) or rejection on day 28 (46%; MC level 1 - 14; MC level 2 - 3; MC level 3 - 2 and >90 recipient chimerism - 4, as per our previous data- Fouzia et al, BMT, 2017). 11/23 (47%) had graft rejection; The OS and EFS were 96% and 79% respectively. Two patients died - 1 class III, of diffuse alveolar hemorrhage/ idiopathic pulmonary syndrome and the other of complications related to primary graft failure. Correlation of PK with all demographic variables by univariate analysis did not reveal any significant associations. However, while 11 out of 39 patients (28%) with 9th dose Bu AUC in the lower three quartiles rejected the graft (Q1: <913 (543-906)-3rej; Q2: 936-1100 -2rej; Q3: 1110-1271- 6rej), none of the 13 patients (0%) in the highest quartile (>1292 (1299-2656) um*min) rejected the graft (p=0.034). While none of the 13 patients in Q4 died, 7 had hepatotoxicity (grade 2 and above) and mucositis (grade 2 and above). The incidence of hepatotoxicity in the first three quartiles were as follows: Q1: 11 had grade 2-3 hepatotoxicity and 2 had grade 2-3 mucositis; in Q2: 5 had grade 2-3 hepatotoxicity while 7 had grade 2-3 mucositis; Q3: 3 had grade 4 hepatotoxicity while 4 had grade 2-3 mucositis (p=ns with respect to toxicity in Q4 vs. the other 3 quartiles). There was no significant association between the busulfan formulation used and the incidence of graft rejection. We then compared the HSCT outcome parameters in these patients with age and Pesaro class matched retrospective cohort of thalassemia patients (n=79) who had received a similar conditioning regimen but with oral Bu without PK guided dose adjustment. There was a significantly better OS (Fig A) in the TBu cohort compared to the oral Bu (p=0.03) but this did not translate to better EFS (Fig B) due to increased incidence of graft rejection (Figure C).
In conclusion, our data suggests that targeting higher Bu AUCs within the therapeutic window could reduce the risk of graft rejection and improve OS without increasing toxicity. Strategies for rapid dose adjustments after the first dose PK are needed to better achieve these target values to reduce rejections and improve outcome.
Figure. Figure.

Disclosures
No relevant conflicts of interest to declare.