Dual CDK4/6–PI3K/mTOR inhibition reinforces cytostatic programs and tumor control in preclinical models of primary and metastatic osteosarcoma

Osteosarcoma (OS) in pediatric, adolescent, and young adult (AYA) patients is an aggressive bone cancer with limited treatment options. Dysregulation of the CDK4/6-cyclin D axis and the PI3K/mTOR pathway contributes to OS pathogenesis, providing a biological rationale for co-targeting these signaling nodes. However, pharmacologic CDK4/6 inhibition can trigger compensatory activation of the PI3K/mTOR pathway, restoring D-type cyclin expression and partially reactivating CDK4/6 signaling. Thus, dual inhibition of the CDK4/6 and PI3K/mTOR pathways not only addresses two parallel oncogenic drivers but may also prevent potential CDK4/6 inhibitor resistance mediated by feedback activation of PI3K/mTOR. In this study, we tested the hypothesis that coordinated targeting of these pathways would improve tumor control in preclinical OS models. In vitro sensitivity analyses using palbociclib and voxtalisib demonstrated additive to synergistic OS growth suppression, with palbociclib inducing G1 arrest and senescence, and the combination enhancing autophagy. Furthermore, the efficacy, tolerability, and mechanisms of palbociclib and voxtalisib, alone or in combination, were evaluated in molecularly defined primary treatment-naïve, and relapsed/metastatic OS models. In the relapsed/metastatic PDX77-TT2 model, short-term palbociclib exposure activated PI3K/mTOR signaling, whereas the combination of palbociclib and voxtalisib in long-term studies produced marked tumor suppression and extended survival. In the primary treatment-naïve PDX96 model, long-term palbociclib exposure generated a robust CDK4/6 pharmacodynamic response. The addition of voxtalisib reinforced autophagy, sustained CDK pathway inhibition, and improved overall tumor control. In an OS lung-colonization model, CDK4/6 inhibition alone markedly reduced OS lung nodules, with combination therapy providing comparable suppression. Dual CDK4/6-PI3K/mTOR inhibition achieves tumor control across various OS models, supporting the use of genomically guided, pathway-targeted strategies for pediatric and AYA OS.