Presentation Authors: Jed Kaminetsky*, New York, NY, Marc Gittelman, Adventura, FL, Ronald Swerdloff, Torrance, CA, James Longstreth, Mundelein, IL, Robert Dudley, Theodore Danoff, Northbrook, IL
Introduction: Measuring T levels during T replacement therapy is critical to guide dosing decisions. Post-collection conversion of TU to T by esterases in blood from men receiving oral TU can raise measured T levels substantially above actual levels and result in inappropriate titration decisions. We identified a method for T monitoring to address this problem.
Methods: We evaluated post-collection conversion of TU to T in blood drawn from men who had received a new oral TU and in blood spiked with TU after collection. Blood was collected in Plain, EDTA or NaF-EDTA tubes and then incubated for periods between 0 and 3 hours, at room temperature (RT) or on ice. After incubation, blood was centrifuged and the matrix (serum or plasma) isolated. T and TU concentrations were measured by LC/MS-MS. Regression analysis of rate of changes of T concentration during incubation v. 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 Clarus&[prime] oral TU.
Results: T concentrations increase in blood samples containing TU as they await centrifugation. The rate of TU to T conversion depends on TU concentration, incubation temperature, and presence of NaF, an esterase inhibitor. Incubation temperature impacted TU to T conversion the most - rate at RT >5-fold faster than on ice; NaF had less effect than temperature. Most clinic T levels are measured in serum; however, titration in the inTUne Trial of oral TU was based on T in NaF-EDTA plasma. Equations were derived to convert the T concentration measured in one matrix to another. Based on regression analysis of T concentrations in serum and NaF-EDTA plasma, 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 (optimal dose-titration sample point for our oral TU). When this conversion factor was tested using T data collected from the inTUne PK visit where both NaF-EDTA plasma and serum were collected, comparing measured serum T values with values derived from plasma T demonstrated a mean error of only 3.1% (N=155; 95% CI 0.4%, 5.8%).
Conclusions: 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 such that T concentrations in our oral TU patients can be monitored using serum T levels. This conversion factor was validated using inTUne data.
Source of Funding: Clarus Therapeutics, Inc.