Background: Duchenne muscular dystrophy (DMD) causes progressive muscle degeneration, inflammation, and fibrosis, leading to severe motor impairment. (Z)-endoxifen, a potent active metabolite of tamoxifen, was evaluated for its ability to improve muscle function in dystrophic mdx5Cv mice.
Methods: Neonatal (P14–P35) and adult (3–6 mo) mdx5Cv mice were randomized and treated with (Z)-endoxifen (1-30mg/kg, subcutaneous injection from 14 to 24 days, or diet supplementation from 25 days to 3 months). Outcomes included body composition (EchoMRI), plasma creatine kinase (CK), and function. Function outcomes were determined by an inverted grid test, accompanied by in situ recording of isometric contraction of the triceps surae from which various kinetic parameters were determined (phasic and tetanic force, time to twitch peak (TTP), and half relaxation time (RT50%). Wild-type C57Bl/6 and mdx5Cv mice treated with vehicle served as controls.
Results: (Z)-endoxifen treatment increased the lean mass and reduced the fat mass relative to body weight by a +2–3% increase and +7–17% decrease, respectively (n=6–8; p=0.016 and 0.0097). In juvenile mice, creatine kinase (CK) levels decreased by +27–47% (n=7–9; p=0.1056), with post hoc multiple comparisons revealing a significant reduction in CK levels in the 3 mg/kg group versus placebo (p=0.0311). Treatment with (Z)-endoxifen also reduced the proportion of centronucleated fibers in the soleus by 30% (n=2–10; p=0.0026). In adult dystrophic mice, (Z)-endoxifen improved motor function to levels comparable to wild-type animals (n=2–6; p<0.0001). Moreover, (Z)-endoxifen fully rescued the absolute phasic force elicited by electrical stimulation (n=3–6; p<0.0001) and altered contractile kinetics, resulting in a 35% longer time to twitch peak (n=3–6; p=0.0015) and a 63% increase in half-relaxation time from the peak (n=3–6; p<0.0001). No adverse findings were observed throughout the duration of the study.
Conclusions: (Z)-endoxifen significantly enhanced muscle strength, endurance, and resistance to contraction-induced injury in mdx5Cv mice, while reducing key biochemical and histologic markers of muscle damage. These findings demonstrate robust preclinical efficacy and provide compelling rationale for clinical translation of (Z)-endoxifen as a novel therapeutic candidate for Duchenne muscular dystrophy, either as monotherapy or in combination with standard of care.