Nanostructured lipid carriers for l-carnitine delivery: physicochemical characterization and effects on asthenozoospermic sperm function in vitro

Asthenozoospermia, driven by oxidative stress and mitochondrial dysfunction, impairs sperm motility and contributes to male infertility. Oral L-carnitine (LC) improves sperm parameters clinically, yet its extreme hydrophilicity severely restricts transmembrane delivery into spermatozoa. However, it remains unknown whether lipid nanocarriers can overcome this previously elusive uptake barrier and augment L-carnitine bioactivity. Here we developed L-carnitine-loaded nanostructured lipid carriers (NLC-LC) and determined their capacity to enhance delivery and functional outcomes in human asthenozoospermic spermatozoa. NLC-LC (size 144 ± 3 nm, zeta - 38.6 ± 1.4 mV, encapsulation efficiency 56 ± 2%) were incubated for 1 h at 0.5 mg/mL LC equivalent with density-gradient-purified spermatozoa from 25 asthenozoospermic men alongside free LC, blank NLC, and untreated controls. NLC-LC demonstrated approximately 34% higher DPPH radical-scavenging activity than free L-carnitine in an acellular assay (47.7 ± 1.1% vs. 35.7 ± 0.9%; P < 0.01). Progressive motility rose from 14.0 ± 5.3% (control) to 20.5 ± 5.5% (free LC; P < 0.05) and 23.9 ± 7.8% (NLC-LC; P < 0.001) versus control, with no significant difference between NLC-LC and free LC (P > 0.05). NLC-LC showed numerically higher viability, lower DNA fragmentation, and higher mitochondrial membrane potential than free LC, but none of these differences reached statistical significance. NLC-LC significantly enhanced antioxidant capacity in the DPPH assay and showed superior numerical performance across all assessed sperm function parameters, although most of these improvements did not reach statistical significance compared with free L-carnitine.