Taming hyper-active Cdk5: Disrupting the Cdk5-p25 axis as a therapeutic avenue for neurodegeneration and beyond.

Cyclin-dependent kinase 5 (Cdk5) is essential for neuronal development and synaptic function when activated by its physiological cofactors p35 and p39. Pathological calpain cleavage of p35 generates the more stable fragment p25, producing a hyperactive, mislocalized kinase complex that has been implicated in tau hyperphosphorylation, DNA damage, neuroinflammation, and aberrant neuronal cell-cycle re-entry. Three decades of work position the Cdk5-p25 axis as a convergent pathogenic mechanism in Alzheimer's disease and related dementias, Parkinson's disease, traumatic brain injury, and in subsets of metabolic and solid-tumor contexts. High-resolution structures of Cdk5-p25 reveal a distinctive activation-loop "cradle" and a tract leading toward catalytic Lys33 that enable structure-guided inhibitor design. Recent advances include (i) small molecules that "vector" toward Lys33 and achieve ∼70-125 × selectivity over closely related CDKs in biochemical assays, (ii) brain-penetrant peptide disruptors that preferentially inhibit Cdk5-p25 while sparing basal Cdk5-p35 signaling in rodent models, and (iii) early-stage degradation or genetic approaches (e.g., dual-target PROTACs; calpain-resistant p35 or Cdk5 knockdown) that reduce p25 signaling or Cdk5 levels in cells. Across inducible mouse models, toxin paradigms, and tumor xenografts, interventions that blunt the p25-driven switch ameliorate cognitive deficits, preserve dopaminergic neurons, improve insulin secretion in β-cell models, and slow tumor growth, respectively. Key translational challenges include achieving durable brain exposure, defining the long-term safety of partial kinase suppression, establishing fluid biomarkers for human studies, and anticipating compensatory signaling. Multiple mechanistic classes are in preclinical development, placing precise disruption of Cdk5-p25 on a credible path toward clinical testing across neurology, oncology, and metabolic disease.