Background: Cutaneous squamous cell carcinoma (cSCC) is a skin cancer that can metastasise in up to 5% of cases. Despite surgery and radiotherapy, survival rates for advanced disease remain low. While immunotherapy has shown remarkable efficacy, many sufferers are ineligible for this therapy; prompting the demand for other interventions. The significance of the PI3K/AKT/mTOR pathway in cSCC has been increasingly reported, although performance of targeted inhibitors has been underwhelming. Combination with cyclin dependent kinase (CDK) inhibitors has been suggested to counter mechanisms of resistance.
Aim: To determine the influence of combination PI3K/CDK inhibitors against metastatic cSCC cell cultures.
Method: PI3K and CDK inhibitors were screened against 2D and 3D metastatic cSCC cultures. Drug combination assays were used to determine synergy/antagonism of select inhibitors at varying concentrations. In response to the optimal concentration range, cell cycle distribution and cell death/signalling responses were determined. Motility effects were determined using a scratch-wound assay. Physiological responses were compared with molecular data obtained from WGS, RNA-seq, NanoString gene expression, and methyl-seq.
Results: Multiple PI3K and CDK inhibitors were found to potently inhibit cell line viability, notably the PI3K inhibitors PIK-75 & BGT226, as well as the CDK inhibitor Dinaciclib. Synergistic drug concentrations for dual treatments (PIK-75 + Dinaciclib or BGT226 + Dinaciclib) were identified for either cell line. Key pathway components were affected significantly in the combination treatment over that of single agent treatments, halting cell cycle progression, inducing apoptosis, and reducing cell motility. These responses correlate with genetic alterations in the PI3K/AKT/mTOR pathway, including: copy number alterations in AKT1, PIK3CA, and PIK3CG, along with numerous single-nucleotide variants – aligning with mutations observed in wider clinical cohorts of metastatic cSCC.
Conclusion: Combination PI3K/CDK inhibitors had a profound impact upon metastatic cSCC. Such treatment may alleviate mechanisms of resistance in vivo, although response may be reliant upon the individual mutational status of the patient.