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Background  Ex vivo androgen prodrug conversion by blood esterases after oral androgen ester administration may result in an overestimation of the measured blood androgens.    Objective  We investigated whether blood collection tubes with esterase inhibitors decreased the conversion of testosterone undecanoate ( TU ) and dimethandrolone undecanoate ( DMAU ) to their active metabolites, testosterone (T), and dimethandrolone ( DMA ), providing a more accurate assessment of circulating T/ DMA  levels.    Methods  Blood was collected in tubes with/without esterase inhibitors from: (i) four healthy and four hypogonadal men receiving no androgens and spiked ex vivo with  TU / DMAU ; (ii) four men taking oral  TU  (Andriol ® ); and (iii) eight hypogonadal men dosed with oral 316 mg  TU  and 15 healthy men with 200 mg  DMAU . T/ DMA  levels were measured by  LC ‐ MS / MS .    Results  Sodium fluoride (NaF, an esterase inhibitor) decreased measured T levels by 14.2% in men not receiving  TU . Increasing amounts of  TU / DMAU  added to blood collected into plain tubes resulted in a concentration‐dependent overestimation of T/ DMA  that was reduced by collecting blood into NaF tubes (by 30–85%), and keeping samples at 4 °C and minimizing time prior to centrifugation. After oral  TU / DMAU  administration to men, when  TU / DMAU  levels were >15/10 ng/mL, respectively, blood collected in NaF tubes yielded lower measured T concentrations by 15–30% and  DMA  by 22% due to an additional inhibitory effect of NaF on blood esterases.    Conclusion  NaF directly lowers plasma T/ DMA  levels measured by  LC ‐ MS / MS  and also inhibits blood esterase activity. Overestimation of T/ DMA  in blood collected in tubes without NaF after oral  TU / DMAU  administration is important for pharmacokinetics studies in drug development clinical trials but may have limited impact in clinical practice/utilization because the differences between measured and true androgen values are modest and the wide therapeutic androgen efficacy ranges obviate the need for highly accurate androgen measurements during therapy.

Accurate measurement of androgen after androgen esters: problems created by ex vivo esterase effects and LC ‐ MS / MS interference