Multiple regression analysis of factors predicting mycophenolic acid free fraction in 91 adult organ transplant recipients

Purpose: Mycophenolic acid (MPA) is an anti-rejection drug used in various types of organ transplants.  MPA is extensively bound to albumin (~97%) and free MPA is thought to be the primary immunosuppressive agent.  Little is known of what contributes to the wide inter-individual variability in the observed MPA free fraction (f%) in humans.  The purpose of this study was to determine, using multiple regression analysis, patient factors that predict f% in a large sample (n = 91) of organ transplant recipients.    

Methods: Age, weight, height, total daily MPA dose, albumin, serum creatinine (SrCr), and f% were obtained from islet (n = 16), kidney (n = 28), and heart/lung (n = 47) transplant recipients.  Multiple linear regression analysis and Spearman Rank correlation were conducted using SigmaStat (version 3.5 for Windows).  Significance was set a priori at p = 0.05. 

Results: The pooled data can be described as (mean ± std): age (52 ± 13 years), weight (72 ± 15 kg), height (169 ± 9 cm), total daily MPA dose (1632 ± 667 mg), albumin (4.2 ± 0.7 g/dL), SrCr (1.3 ± 0.4 mg/dL), and f% (2.9 ± 3.5%).  Multiple regression generated the following equation:  f% = 1.865 + (0.0357 x age (yr)) + (0.0125 x weight (Kg)) - (0.0202 x height (cm)) - (0.000323 x total daily dose (mg)) + (0.0122 x albumin (g/L)) + (0.0160 x SrCr (µmol/L)),(r²= 0.06), but none of the variables were significant predictors of MPA f% (p > 0.05).  Spearman Rank correlation of each individual variable confirmed lack of significant correlation with f%.   

Conclusion: To our knowledge, this is the first study attempting to describe factors predicting MPA f% in organ transplant recipients involving a large sample size.  Our novel findings of lack of significant predictions warrant further investigations using additional patient factors.