Development of TSSK1 and TSSK2 Targeted Degraders Reveals Sperm Machinery for Protein Degradation and Potential for Nonhormonal Male Contraception

We report the design, synthesis, and characterization of a series of targeted degraders directed against testis-specific serine/threonine kinases TSSK1 and TSSK2. A stable CHO-K1 cell line expressing HiBiT-tagged TSSK1 enabled quantitative cellular profiling and identified compound 5.1 as a potent TSSK1 degrader with a DC₅₀ of 10 nM and a D_max of 68% after 48 h. In contrast, ex vivo incubation of CD1 mouse sperm with degrader 5.4 induced up to an 80% reduction of the TSSK2 isoform within 4 h and translated into profound functional loss, resulting in a 97% reduction in sperm motility and near-complete loss of in vitro fertilization. Both alkyl- and PEG-linked degraders displayed metabolic N-dealkylation and high efflux, and therefore will require optimization. These data provide proof of concept that targeted degradation can occur within sperm cells, leading to a disruption of sperm function and encouraging further optimization toward in vivo evaluation for male contraception.