Cyclophosphamide Kinetics Influences Toxicity and Outcome of Bone Marrow Transplantation in Patients with β-Thalassemia Major.
Authors: Srivastava Alok , Poonkuzhali Balasubramanian , Salamun Desire , George Biju , Mathews Vikram , Krishnamoorthy Rajagopal , Chandy Mammen 
Published: October 2019

Busulfan and cyclophosphamide (Cy) are commonly used in conditioning regimen for bone marrow transplantation (BMT). Cy, a prodrug, undergoes hepatic biotransformation to 4-hydroxy cyclophosphamide (4-HCy) and subsequently to its active metabolite, phosphoramide mustard (PM). Though toxic complications like hemorrhagic cystitis (HC) and hepatic veno occlusive disease (HVOD) have been associated with metabolites of Cy such as acrolein and carboxyethylPM, respectively, there is no data correlating pharmacokinetics (PK) of Cy or HCy with toxicity and outcome of BMT for thalassemia major. We analyzed PK of Cy and HCy in 40 patients with β- thalassemia major (Class II- 20; class III-20) undergoing BMT at our centre from January 2001 to February 2004. Cy was administered at 50mg/kg/day for 4 days (days -5 to -2) following 4 days of busulfan (days -9 to -6). Levels of Cy and 4-HCy were measured by high performance liquid chromatography and correlated with the incidences of HVOD (McDonald’s criteria), HC, transplant related mortality (TRM) and graft rejection (rej). PK of Cy and HCy showed 4–20-fold inter individual variation. There was a significantly shorter half-life of 4HCy in patients who developed HVOD compared to those who did not. This suggests that rapid conversion of HCy to its metabolites increases toxicity to hepatic sinusoidal cells. In patients with HC, there was significantly shorter half-life of Cy and significantly faster clearance of Cy. In the 10 patients who had TRM, AUC of HCy was significantly low and there was a trend towards lower ratio of AUC HCy/Cy.

Association of Cy PK parameters with BMT outcome and toxicity

Cy AUC1 μ g*L/h) ( Cy T1/2-1 (hours) Cy Cl1(l/h/kg) HCy AUC1(μg*L/h) HCy T1/2-1(hours) AUC1 HCy/Cy ratio HVOD +(n=19) 2103±1781 4.8±2.27 0.0294±0.016 4.9±2.5 HVOD -(n=21) 2593±1127 5.2±2.3 0.0238±0.009 5.9±2.4 3.71±1.38 0.0029±0.0017 p value 0.145 0.486 0.41 0.229 0.004 0.95 HC +(n=14) 2254±1236 3.94±1.7 0.031±0.01 5.26±1.7 2.7±1.2 0.002±0.001 HC -(n=26) 2380±1152 5.63±2.3 0.024±0.015 5.89±2.83 3.2±1.6 0.003±0.002 p value 0.585 0.05 0.049 0.812 0.474 0.685 TRM + (n=10) 2408±857 4.02±2.09 .023±0.012 4.14±1.54 2.58±0.9 0.0021±0.0013 TRM - (n=30) 2312±1266 4.2±1.8 0.028±0.014 6.18±2.5 3.19±1.6 0.0032±0.0017 p value 0.528 0.988 0.43 0.014 0.363 0.072 Rej + (n=4) 2074±1146 4.27±1.7 0.016±0.01 3.8±2.07 2.3±1.06 0.0022±0.0018 Rej - (n=26) 2365±1182 414±1.9 0.028±0.014 5.87±2.5 3.1±1.5 0.0033±0.0017 p value 0.586 0.419 0.05 0.161 0.419 0.262

These observations suggest that faster biotransformation of Cy to its metabolites contributes to increased toxicity. Four of these patients rejected their grafts, two being primary graft failure. Clearance of Cy was significantly slower in them, suggesting that slower conversion of the active metabolites may contribute to graft rejection. Our data shows that PK analysis of Cy can be helpful in predicting outcome and toxicity after BMT for thalassemia major.