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Androgen Receptor Gene CAG Repeat Length and Body Mass Index Modulate the Safety of Long-Term Intramuscular Testosterone Undecanoate Therapy in Hypogonadal Men

Institute of Reproductive Medicine, University Clinics, Muenster D-48149, Germany

Address all correspondence and requests for reprints to: Eberhard Nieschlag, Institute of Reproductive Medicine of the University, Muenster D-48149, Germany. E-mail: eberhard.nieschlag{at}ukmuenster.de.

Context: A reliable form of androgen substitution therapy regarding kinetics, tolerance, and restoration of androgenicity is paramount in hypogonadal men. Intramuscular injection of the long-acting ester testosterone undecanoate (TU) offers a new modality.

Objective: The objective of the study was to assess the safety of TU regarding metabolic and pharmacogenetic confounders.

Design: This was a longitudinal one-arm open observation trial. A minimum of five individual assessments was a prerequisite. Putative modulators of safety parameters entering regression models were nadir and/or delta total testosterone concentrations, body mass index, androgen receptor (AR) gene CAG repeat length, and age.

Setting: The study was conducted at an andrological outpatient clinic.

Patients: Patients included 66 hypogonadal men (mean age 38 ± 9.9 yr).

Main Outcome Measures: A total of 515 data time points each related to prostate, erythropoiesis, lipoproteins, and circulation during 118 treatment-years with 1000 mg TU at 10- to 14-wk intervals.

Results: Testosterone substitution resulted in significant decrements of serum levels of low-density lipoprotein-cholesterol, resting diastolic and systolic blood pressure, and heart rate. Erythropoiesis was stimulated and concentrations of high-density lipoproteincholesterol increased. Parameters remained stable after four injections. No adverse effects regarding the prostate were observed. Significantly increased hematocrit greater than 50% was predicted by enhanced androgen action (shorter AR CAG repeats per higher testosterone levels). However, insufficient androgen action (longer AR CAG repeats per lower testosterone levels) caused pathological safety parameters (high blood pressure, adverse lipid profiles). In addition, a body mass index 30 kg/m² or greater represents a clinically relevant factor for the occurrence of all pathological safety parameters. Risk calculations for obese patients and nonlinear pharmacogenetic models to tailor androgen substitution are presented.

Conclusions: Testosterone substitution with im TU is generally well tolerated. Modifications of androgen action are due to both AR CAG repeats and testosterone levels. Adverse observations are mostly seen in obese patients.