Detection time comparison of non-hydrolysed sulphated metabolites of metenolone, mesterolone and 17α-methyltestosterone analysed by four different mass spectrometric techniques.

The frequent detection of anabolic androgenic steroids (AAS) indicates their popularity among rule-breaking athletes. The so called long-term metabolites play a crucial role in their detection, and non-hydrolysed sulphated metabolites have gained renewed interest, as research has demonstrated their extended detection time compared to the more conventional markers (e.g., for metenolone and mesterolone). Their potential has been investigated using liquid and gas chromatography-mass spectrometry (LC- and GC-MS). However, due to their complementary nature, chances are that the most promising metabolite on one technique does not necessarily exhibit the same behaviour on the other and vice versa. Therefore, a comparison was carried out where as a trial model, metenolone, mesterolone and 17α-methyltestosterone were selected and the most likely long-term sulphated metabolites identified on four mass spectrometric instruments. Additionally, using a modified sample preparation procedure, comparison between conventional and non-hydrolysed sulphated metabolites between different GC-MS instruments was also included. When focusing on each individual marker, no cases were observed where a single metabolite provided a superior detection time on all instruments. Furthermore, for each AAS, there were incidences where a metabolite provided the best detection time on one instrument but could only be detected for a shorter period or not at all on other instruments. This demonstrates that metabolite detection windows and hence their added-value as target substance are unique and dependent on the analytical technique and not only on their pharmacokinetic behaviour. Consequently, in each case, a metabolite versus instrument evaluation is needed to maximise the probabilities of detecting doping offences.

Glomerella fusarioides-catalyzed structural transformation of steroidal drugs mesterolone and methasterone, and anti-inflammatory activity of resulting derivatives.

Transformation of steroidal drug mesterolone (1) with Glomerella fusarioides yielded two new (17α-hydroxy-1α-methyl-5α-androstan-3-one-11α-yl acetate (2) and 15α-hydroxy-1-methyl-5α-androstan-1-en-3,17-dione (3)), and four known derivatives (15α,17ß-dihydroxy-1α-methyl-5α-androstan-3-one (4), 15α-hydroxy-1α-methyl-5α-androstan-3,17-dione (5), 1α-methyl-androsta-4-en-3,17-dione (6) and 15α,17ß-dihydroxy-1-methyl-5α-androstan-1-en-3-one (7). Similarly, G. fusarioides-catalyzed transformation of steroidal drug methasterone (8) afforded four new metabolites, 11α,17ß-dihydroxy-2,17α-dimethylandrosta-1,4-diene-3-one (9), 3a,11α,17ß-trihydroxy-2α,17α-dimethyl-5α-androstane (10), 1ß,3ß,17ß-trihydroxy-2α,17α-dimethyl-5α-androstane (11), and 11α,17ß-dihydroxy-2,17α-dimethylandrosta-1,4-diene-3-one (12). Structures of new derivatives were determined by using 1D-, and 2D-NMR, HREI-MS, and IR spectroscopic data. New derivative 3 was identified as a potent inhibitor of NÈ® production with the IC50 value of 29.9 ± 1.8 µM, in comparison to the standard l-NMMA (IC50 = 128.2 ± 0.8 µM) in vitro. In addition, methasterone (8) (IC50 = 83.6 ± 0.22 µM) also showed a significant activity comparable to new derivative 12 (IC50 = 89.8 ± 1.2 µM). New derivatives 2 (IC50 = 102.7 ± 0.5 µM), 9 (IC50 = 99.6 ± 5.7 µM), 10 (IC50 = 123.5 ± 5.7 µM), and 11 (IC50 = 170.5 ± 5.0 µM) showed a moderate activity. NG-MonomethylL-arginine acetate (IC50 = 128.2 ± 0.8 µM) was used as standared NO⋅- free radicals have an important role in the regulation of immune responses and cellular events. Their overproduction is associated with the pathogenesis of numerous ailments, such as Alzheimer's cardiac disorders, cancer, diabetes, and degenerative diseases. Therefore, inhibition of NÈ® production can help in the treatment of chronic inflammation and associated disorders. All derivatives were found to be non-cytotoxic to human fibroblast (BJ) cell line. The results presented here form the basis of further research for the development of new anti-inflammatory agents with improved efficacy through biotransformation approaches.

LC-MS/(MS) confirmatory doping control analysis of intact phase II metabolites of methenolone and mesterolone after Girard's Reagent T derivatization.

In the present study, the application and evaluation of Girard's Reagent T (GRT) derivatization for the simultaneous detection and significantly important identification of different phase II methenolone and mesterolone metabolites by LC-MS/(MS) are presented. For the LC-MS analysis of target analytes two complementary isolation methods were developed; a derivatization and shoot method in which native urine is diluted with derivatization reagent and is injected directly to LC-MS and a liquid-liquid extraction method, using ethyl acetate at pH 4.5, for the effective isolation of both sulfate and glucuronide metabolites of the named steroids as well as of their free counterparts. For the evaluation of the proposed protocols, urine samples from methenolone and mesterolone excretion studies were analyzed against at least one sample from a different excretion study. Retention times, along with product ion ratios, were evaluated according to the WADA TD2021IDCR requirements, in order to determine maximum detection and identification time windows for each metabolite. Established identification windows obtained after LC-MS/(MS) analysis were further compared with those obtained after GC-MS/(MS) analysis of the same samples from the same excretion studies, for the most common analytes monitored by GC-MS/(MS). Full validation was performed for the developed derivatization and shoot method for the identification of methenolone metabolite, 3α-hydroxy-1-methylen-5α-androstan-17-one-3-glucuronide (mth3). Overall, the GRT derivatization presented herein offers a tool for the simultaneous sensitive detection of free, intact glucuronide and sulfate metabolites by LC-MS/(MS) that enhance significantly the detection and identification time windows of specific methenolone and mesterolone metabolites for doping control analysis.

Differential response of oxidative and glycolytic skeletal muscle fibers to mesterolone.

Oxidative and glycolytic muscle fibers differ in their ultrastructure, metabolism, and responses to physiological stimuli and pathological insults. We examined whether these fibers respond differentially to exogenous anabolic androgenic steroids (AASs) by comparing morphological and histological changes between the oxidative anterior latissimus dorsi (ALD) and glycolytic pectoralis major (PM) fibers in adult avian muscles. Adult female White Leghorn chickens (Gallus gallus) were randomly divided into five groups: a vehicle control and four mesterolone treatment groups (4, 8, 12, and 16 mg/kg). Mesterolone was administered orally every three days for four weeks. Immunocytochemical techniques and morphometric analyses were employed to measure the changes in muscle weight, fiber size, satellite cell (SC) composition, and number of myonuclei. Mesterolone increased both body and muscle weights and induced hypertrophy in glycolytic PM fibers but not in oxidative ALD fibers. Mesterolone induced SC proliferation in both muscles; however, the myonuclear accretion was noticeable only in the PM muscle. In both muscles, the collective changes maintained a constant myonuclear domain size and the changes were dose independent. In conclusion, mesterolone induced distinct dose-independent effects in avian oxidative and glycolytic skeletal muscle fibers; these findings might be clinically valuable in the treatment of age-related sarcopenia.

Liquid chromatography-mass spectrometry behavior of Girard's reagent T derivatives of oxosteroid intact phase II metabolites for doping control purposes.

Intact phase II steroid metabolites have poor product ion mass spectra under collision-induced dissociation (CID) conditions. Therefore, we present herein the liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/(MS)) behavior of intact phase II metabolites of oxosteroids after derivatization. Based on the fact that Girard's reagent T (GRT), as derivatization reagent, was both convenient and efficient in terms of the enhancement in the ionization efficiency and the production of diagnostic product ions related to the steroid moiety, the latter was preferably selected between methoxamine and hydroxylamine upon the model compounds of androsterone glucuronide and androsterone sulfate. Sixteen different glucuronides and 29 sulfate conjugated metabolites of anabolic androgenic steroids (AASs), available either as pure reference materials or synthesized/extracted from administration studies, were derivatized with GRT, and their product ion spectra are presented. Product ion spectra include in all cases high number of product ions that in some cases are characteristic for certain structures of the steroid backbone. More specifically, preliminary results have shown major differences in fragmentation pattern for 17α/17ß-isomers of the sulfate conjugates, but limited differentiation for 17α/17ß-isomers of glucuronide conjugates and for 3α/3ß- and 5α/5ß-stereoisomers of both sulfate and glucuronide conjugates. Further to the suggestion of the current work, application on mesterolone administration studies confirmed-according to the World Anti-Doping Agency (WADA) TD2015IDCR-the presence of seven intact phase II metabolites, one glucuronide and six sulfates with use of LC-ESI-MS/(MS).

Daño hepático inducido por mesterolona: a propósito de un caso / Drug-induced liver injury due to mesterolone: A case report

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Gas chromatography-mass spectrometry analysis of non-hydrolyzed sulfated steroids by degradation product formation.

Steroid detection and identification remain key issues in toxicology, drug testing, medical diagnostics, food safety control, and doping control. In this study, we evaluate the capabilities and usefulness of analyzing non-hydrolyzed sulfated steroids with gas chromatography-mass spectrometry (GC-MS) instead of the conventionally applied liquid chromatography-mass spectrometry (LC-MS) approach. Sulfates of 31 steroids were synthesized and their MS and chromatographic behavior studied by chemical ionization-GC-triple quadrupole MS (CI-GC-TQMS) and low energy-electron ionization-GC-quadrupole time-of-flight-MS (LE-EI-GC-QTOF-MS). The collected data shows that the sulfate group is cleaved off in the injection port of the GC-MS, forming two isomers. In CI, the dominant species (ie, [MH - H2 SO4 ]+ or [MH - H4 S2 O8 ]+ for bis-sulfates) is very abundant due to the limited amount of fragmentation, making it an ideal precursor ion for MS/MS. In LE-EI, [M - H2 SO4 ].+ and/or [M - H2 SO4 - CH3 ].+ are the dominant species in most cases. Based on the common GC-MS behavior of non-hydrolyzed sulfated steroids, two applications were evaluated and compared with the conventionally applied LC-MS approach; (a) discovery of (new) sulfated steroid metabolites of mesterolone and (b) expanding anabolic androgenic steroid abuse detection windows. GC-MS and LC-MS analysis of non-hydrolyzed sulfated steroids offered comparable sensitivities, superseding these of GC-MS after hydrolysis. For non-hydrolyzed sulfated steroids, GC-MS offers a higher structural elucidating power and a more straightforward inclusion in screening methods than LC-MS.

Chronic Exercise Reduces CETP and Mesterolone Treatment Counteracts Exercise Benefits on Plasma Lipoproteins Profile: Studies in Transgenic Mice.

Regular exercise and anabolic androgenic steroids have opposing effects on the plasma lipoprotein profile and risk of cardio-metabolic diseases in humans. Studies in humans and animal models show conflicting results. Here, we used a mice model genetically modified to mimic human lipoprotein profile and metabolism. They under-express the endogenous LDL receptor gene (R1) and express a human transgene encoding the cholesteryl ester transfer protein (CETP), normally absent in mice. The present study was designed to evaluate the independent and interactive effects of testosterone supplementation, exercise training and CETP expression on the plasma lipoprotein profile and CETP activity. CETP/R1 and R1 mice were submitted to a 6-week swimming training and mesterolone (MEST) supplementation in the last 3 weeks. MEST treatment increased markedly LDL levels (40%) in sedentary CETP/R1 mice and reduced HDL levels in exercised R1 mice (18%). A multifactorial ANOVA revealed the independent effects of each factor, as follows. CETP expression reduced HDL (21%) and increased non-HDL (15%) fractions. MEST treatment increased the VLDL concentrations (42%) regardless of other interventions. Exercise training reduced triacylglycerol (25%) and free fatty acids (20%), increased both LDL and HDL (25-33%), and reduced CETP (19%) plasma levels. Significant factor interactions showed that the increase in HDL induced by exercise is explained by reducing CETP activity and that MEST blunted the exercise-induced elevation of HDL-cholesterol. These results reinforce the positive metabolic effects of exercise, resolved a controversy about CETP response to exercise and evidenced MEST potency to counteract specific exercise benefits.

Serologically silent, occult equine infectious anemia virus (EIAV) infections in horses.

Molecular and serological techniques for Equine Infectious Anemia Virus (EIAV) diagnosis were compared using samples from 59 clinically normal horses stabled on five farms in the Santa Fe Province of Argentina. Of these 26 (44.1%) were positive in official AGID tests and/or gp45/gp90-based ELISA. Surprisingly 18 of the 33 seronegative horses were positive in a PCR against viral sequences encoding gp45 (PCR-positive/AGID-negative) with all but one remaining EIAV-antibody negative throughout a two year observation period. The gp45 PCR results are supported by fact that 7/18 of these horses were positive in the Office International des Epizooties (OIE) recommended EIAV gag gene specific PCR plus 2 of this 7 also reacted in a PCR directed predominantly against the 5' untranslated region of the viral genome. Furthermore sufficient quantities of serum were available from 8 of these horses to verify their seronegative status in sensitive Western Blot tests and demonstrate by ELISA the absence of EIAV-specific antibodies was not attributable to abnormalities in total IgG concentration. Studies involving 7 of the PCR-positive/AGID-negative horses to measure lymphocyte proliferation in the presence of PHA showed no significant differences between this group and control animals. In addition, lymphocytes from 2 of these 7 horses responded to peptides derived from gp90 and gp45. Together these results demonstrate that apparently clinically normal horses with no gross signs of immunodeficiency in terms of total IgG concentration or T helper-cell function can remain seronegative for at least 24 months while harboring EIAV specific nucleic acid sequences.

Analysis of anabolic androgenic steroids as sulfate conjugates using high performance liquid chromatography coupled to tandem mass spectrometry.

Improvements in doping analysis can be effected by speeding up analysis time and extending the detection time. Therefore, direct detection of phase II conjugates of doping agents, especially anabolic androgenic steroids (AAS), is proposed. Besides direct detection of conjugates with glucuronic acid, the analysis of sulfate conjugates, which are usually not part of the routine doping control analysis, can be of high interest. Sulfate conjugates of methandienone and methyltestosterone metabolites have already been identified as long-term metabolites. This study presents the synthesis of sulfate conjugates of six commonly used AAS and their metabolites: trenbolone, nandrolone, boldenone, methenolone, mesterolone, and drostanolone. In the following these sulfate conjugates were used for development of a fast and easy analysis method based on sample preparation using solid phase extraction with a mixed-mode sorbent and detection by high performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS). Validation demonstrated the suitability of the method with regard to the criteria given by the technical documents of the World Anti-Doping Agency (WADA). In addition, suitability has been proven by successful detection of the synthesized sulfate conjugates in excretion urines and routine doping control samples.

New potential biomarkers for mesterolone misuse in human urine by liquid chromatography quadrupole time-of-flight mass spectrometry.

In this paper, mesterolone metabolic profiles were investigated carefully. Mesterolone was administered to one healthy male volunteer. Urinary extracts were analyzed by liquid chromatography quadruple time-of-flight mass spectrometry (LC-QTOFMS) for the first time. Liquid-liquid extraction was applied to processing urine samples, and dilute-shoot analyses of intact metabolites were also presented. In LC-QTOFMS analysis, chromatographic peaks for potential metabolites were hunt down by using the theoretical [M-H](-) as target ions in full scan experiment, and their actual deprotonated ions were analyzed in targeted MS/MS mode. Ten metabolites including seven new sulfate and three glucuronide conjugates were found for mesterolone. Because of no useful fragment ion for structural elucidation, gas chromatography-mass spectrometry instrumentation was employed to obtain structural details of the trimethylsilylated phase I metabolite released after solvolysis. Thus, their potential structures were proposed particularly by a combined MS approach. All the metabolites were also evaluated in terms of how long they could be detected, and S1 (1α-methyl-5α-androst-3-one-17ß-sulfate) together with S2 (1α-methyl-5α-androst-17-one-3ß-sulfate) was detected up to 9 days after oral administration, which could be the new potential biomarkers for mesterolone misuse.

A Validated TLC-Densitometric Method for the Determination of Mesterolone in Bulk Material and in Tablets.

Mesterolone is a synthetic androgenic steroid indicating a weak anabolic activity. A new, simple in use, and economical TLC-densitometric method in normal phase system (NP-TLC) has been developed and validated for the identification and quantitative determination of mesterolone in bulk drug and in tablet formulation. NP-TLC analysis was performed on aluminium plates precoated with silica gel 60F254 as the stationary phase using chloroform-acetone (40 : 10, v/v) as mobile phase. Densitometric analysis was carried out at λ = 745 nm after staining with phosphomolybdic acid. These conditions were found to give visible (dark blue) spot and sharp peak, respectively, for mesterolone at R F 0.75 ± 0.02 and enabled satisfactory separation of mesterolone from its related substance (potential impurity). The proposed NP-TLC-densitometric method was validated for specificity, linearity, precision, accuracy, robustness, and sensitivity according to ICH guideline and other validation requirements. The limit of detection (LOD) and limit of quantification (LOQ) were 61.0 ng · spot(-1) and 184.0 ng · spot(-1), respectively. The percent content of mesterolone in marketed tablet formulation was found to be 99.40% of label claim. The developed TLC-densitometric method can be successfully used in quality control of mesterolone in bulk material and also tablet formulation.

Biotransformation of androgenic steroid mesterolone with Cunninghamella blakesleeana and Macrophomina phaseolina.

Fermentation of mesterolone (1) with Cunninghamella blakesleeana yielded four new metabolites, 1α-methyl-1ß,11ß,17ß-trihydroxy-5α-androstan-3-one (2), 1α-methyl-7α,11ß,17ß-trihydroxy-5α-androstan-3-one (3), 1α-methyl-1ß,6α,17ß-trihydroxy-5α-androstan-3-one (4) and 1α-methyl-1ß,11α,17ß-trihydroxy-5α-androstan-3-one (5), along with three known metabolites, 1α-methyl-11α,17ß-dihydroxy-5α-androstan-3-one (6), 1α-methyl-6α,17ß-dihydroxy-5α-androstan-3-one (7) and 1α-methyl-7α,17ß-dihydroxy-5α-androstan-3-one (8). Biotransformation of 1 with Macrophomina phaseolina also yielded a new metabolite, 1α-methyl, 17ß-hydroxy-5α-androstan-3,6-dione (9). The isolated metabolites were subjected to various in vitro biological assays, such as anti-cancer, inhibition of α-glucosidase, and phosphodiesterase-5 enzymes and oxidative brust. However, no significant results were observed. This is the first report of biotransformation of 1 with C. blakesleeana and M. phaseolina.

Mesterolone treatment of aging male syndrome improves lower urinary tract symptoms.

OBJECTIVE: To investigate the effects of mesterolone on prostate in patients treated for aging male syndrome. METHODS: The cross-sectional study was conducted from June to September, 2009, at endocrinology and metabolism department of Yuzuncu Yil University, Van, Turkey, andcomprised patients with symptoms of aging male syndrome and/or low testosterone. They were given mesterolone 50 mg/day per oral for two months. Aging Male Symptoms and International Prostate Symptom Score questionaires and prostate-related quality of life scores were completed and prostate ultrasonography (USG) was performed before and after the treatment. Total testosterone, free testosterone, gonadotropins, estradiol, prolactin, sex-hormone binding globulin, as well as total and free prostate-specific antigen were also studied. RESULTS: Of the 34 patients in the study, 22(64.70%) had their prostate volume increased, while 12(35.29%) had it decreased. The change, however, was not statistically significant (p<0.098). Mesterolone significantly improved Aging Male Symptoms, International Prostate Symptom and prostate-related quality of life scores (p<0.001). These improvements though significant were independent of the changes in prostate volume. Total testosterone, sex-hormone binding globulin andestradiol decreased, while free testosterone showed no change (p<0.002, p<0.001, p<0.024, p<0.337). The fraction of free testosterone increased (p<0.001), while total and free prostate-specific antigen did not change (p<0.368 and p<0.841) CONCLUSION: Mesterolone proved to be a safe alternative in the treatment of Aging Male Syndrome. It also improved lower urinary tract symptoms and prostate-related quality of life.

Effects of anabolic steroids and high-intensity aerobic exercise on skeletal muscle of transgenic mice.

In an attempt to shorten recovery time and improve performance, strength and endurance athletes occasionally turn to the illicit use of anabolic-androgenic steroids (AAS). This study evaluated the effects of AAS treatment on the muscle mass and phenotypic characteristics of transgenic mice subjected to a high-intensity, aerobic training program (5d/wk for 6 weeks). The transgenic mice (CETP(+/-)LDLr(-/+)) were engineered to exhibit a lipid profile closer to humans. Animals were divided into groups of sedentary (Sed) and/or training (Ex) mice (each treated orally with AAS or gum arabic/vehicle: Sed-C, Sed-M, ex-C, ex-M). The effects of AAS (mesterolone: M) on specific phenotypic adaptations (muscle wet weight, cross-sectional area, and fiber type composition) in three hindlimb muscles (soleus:SOL, tibialis anterior:TA and gastrocnemius:GAS) were assessed. In order to detect subtle changes in fiber type profile, the entire range of fiber types (I, IC, IIAC, IIA, IIAD, IID, IIDB, IIB) was delineated using mATPase histochemistry. Body weight gain occurred throughout the study for all groups. However, the body weight gain was significantly minimized with exercise. This effect was blunted with mesterolone treatment. Both AAS treatment (Sed-M) and high-intensity, aerobic training (ex-C) increased the wet weights of all three muscles and induced differential hypertrophy of pure and hybrid fibers. Combination of AAS and training (ex-M) resulted in enhanced hypertrophy. In the SOL, mesterolone treatment (Sed-M and ex-M) caused dramatic increases in the percentages of fiber types IC, IIAC, IIAD, IID, with concomitant decrease in IIA, but had minimal impact on fiber type percentages in the predominantly fast muscles. Overall, the AAS-induced differential adaptive changes amounted to significant fiber type transformations in the fast-to-slow direction in SOL. AAS treatment had a significant effect on muscle weights and fiber type composition in SOL, TA and GAS which was even maximized in animals subjected to metabolically high-intensity aerobic exercise.

Regulation of neuronal and endothelial nitric oxide synthase by anabolic-androgenic steroid in skeletal muscles.

Anabolic-androgenic steroids (AAS) and exercise share comparable effects on myogenic differentiation, force development, fiber growth and skeletal muscle plasticity. The participation of nitric oxide synthase (NOS) on these effects was only demonstrated in response to exercise. Using immunohistochemistry and western blotting we examined the effect of AAS on the expression of NOS I and III isoforms in three muscles, distinct metabolically and physiologically: soleus (SOL), tibialis anterioris (TA) and gastrocnemius (GAS). Mice with a lipid profile akin to humans were used. Sedentary mice (Sed-C) or exercised, submitted to six-weeks of aerobic treadmill running (one hour/day, 5 days/week) were administered mesterolone (Sed-M and Ex-M, respectively) or gum arabic (vehicle, Ex-C) during the last three weeks, three alternate days per week. Consistently, The TA showed the strongest labeling and the SOL the weakest with NOS III predominating over NOS I. Mesterolone administered to sedentary mice (Sed-C x Sed-M) significantly upregulated NOS I in TA and SOL and NOS III in all three muscles. Mesterolone administered to exercised mice (Ex-C x Ex-M) upregulated NOS I in all three muscles and NOS III in TA and SOL. The exercise to mesterolone-treated mice (Sed-M x Ex-M) produced a strong increase in NOS I expression in GAS; in contrast it antagonized the mesterolone-induced upregulation of NOS I in TA muscle and NOS III in SOL and GAS. The data show nitric oxide (NO) as a potential signaling mediator of AAS effects in skeletal muscle and that NOS I and NOS III upregulations were muscle phenotype-specific. These may be regarded as an indication of the complex NOS/NO signaling mechanism related with AAS effects vs. metabolic/physiological muscle characteristics.

Influence of mesterolone on satellite cell distribution and fiber morphology within maturing chicken pectoralis muscle.

Mesterolone is a synthetic oral anabolic androgenic steroid used to treat hypogonadism. There are frequent reports of mesterolone abuse in human and equine sports to increase muscle mass and strength. However, limited information is available about how this drug exerts its effects on skeletal muscle. Satellite cells (SCs) are mononuclear myogenic stem cells that contribute to postnatal muscle growth and repair. As SC activation and subsequent differentiation to new myonuclei is a major event during muscle hypertrophy, this study investigated the influence of mesterolone on SC distribution within the pectoralis muscle of chickens. Specifically, this study tested the hypotheses that mesterolone induces avian skeletal muscle hypertrophy, and that mesterolone increases the number of SCs in avian skeletal muscle. Robust immunocytochemical techniques and morphometric analyses were used to calculate the numbers of SCs and myonuclei. Also, DNA concentration and Pax7 protein levels were measured to confirm immunocytochemical findings. Mesterolone significantly increased pectoralis mass and fiber size. All SC indices and number of myonuclei increased significantly by mesterolone administration. In addition, greater DNA concentration and Pax7 protein expression were found in mesterolone-treated birds. This study indicates that mesterolone can induce avian skeletal muscle hypertrophy and that this is correlated with increased number of SCs. We suggest that SCs are key cellular intermediaries for mesterolone-induced muscle hypertrophy.