SUN-227 Monitoring Testosterone (T) Levels in Men Receiving Oral Testosterone Undecanoate (TU): Challenges Due to Post-Collection Conversion of TU-to-T and a Path Forward

Introduction: Measuring T levels, in addition to monitoring symptoms, during T replacement therapy is critical to guide dosing decisions. T levels in blood samples from men receiving oral TU can be elevated substantially above actual circulating T levels due to post-collection conversion of TU to T by esterases in blood. Because erroneous T values can result in inappropriate dose-titration decisions, we identified a method for monitoring T that addresses this problem. Methods: Post-collection conversion of TU to T was evaluated in blood drawn from men who had received oral TU (either JATENZO®, Clarus’s oral TU, or Andriol®) and in blood spiked with TU after collection. Blood was collected in Plain, EDTA or NaF-EDTA tubes and then incubated for selected times (up to 3 hours) at room temperature (RT) or on ice. After incubation, blood was centrifuged and the matrix (serum or plasma) was used for measurement of T and TU concentrations by LC/MS-MS. Regression analysis of rate of change of T concentration during incubation versus TU concentration was used to develop algorithms to correct for T overestimation. Algorithm accuracy was tested using results from the Phase 3 inTUne Trial of JATENZO. Results: T concentrations increase in blood containing TU as the blood sits pre-centrifugation, regardless of whether the TU is in the blood when collected or spiked post-collection. The rate of TU conversion depends on the TU concentration, incubation temperature, and presence of NaF, an esterase inhibitor. Incubation temperature had the greatest impact on conversion - rate at RT >5-fold faster than on ice; NaF had a smaller effect. Most clinic T levels are routinely measured in serum from Plain tubes; however, titration in the inTUne Trial of oral TU was based on T in NaF-EDTA plasma. Based on regression analysis of the TU to T conversion rates measured in serum and NaF-EDTA plasma and the NaF effect on measured T levels, a conversion factor of 1.214 was derived to convert a NaF-EDTA plasma T value to an equivalent serum T value for samples collected 6 hours post-dose (the optimal dose-titration sample point for JATENZO). This conversion factor was tested against T data collected during the final PK Visit of the inTUne Trial (87% of subjects attained eugonadal range based on NaF-EDTA plasma T levels) where serum T levels were also measured. Using the conversion factor to compare the measured serum T value with its matched plasma value, we observed a mean error of only 3.1% (n=155 sample pairs; 95% CI 0.4%, 5.8%). Conclusion: Post-collection conversion of TU to T can cause overestimation of circulating T levels in men dosed with oral TU. By accounting for the conversion with different tube types / handling conditions, a conversion factor was derived to allow monitoring of T concentration in JATENZO patients using serum. This conversion factor was validated for JATENZO against the Phase 3 inTUne data.