Methods: The effect of palbociclib was evaluated in a panel of well-characterized OSCC cell lines by cell proliferation assays and further confirmed by in vivo evaluation in xenograft models. PIK3CA-mutant isogenic cell lines were used to investigate the effect of PIK3CA mutation towards palbociclib response.
Results: We demonstrated that 80% of OSCC cell lines are sensitive to palbociclib at sub-micromolar concentrations. Consistently, palbociclib was effective in controlling tumor growth in mice. We identified that palbociclib-resistant cells harbored mutations in PIK3CA. Using isogenic cell lines, we showed that PIK3CA mutant cells are less responsive to palbociclib as compared to wild-type cells with concurrent upregulation of CDK2 and cyclin E1 protein levels. We further demonstrated that the combination of a PI3K/mTOR inhibitor (PF-04691502) and palbociclib completely controlled tumor growth in mice.
Conclusions: This study demonstrated the potency of palbociclib in OSCC models and provides a rationale for the inclusion of PIK3CA testing in the clinical evaluation of CDK4/6 inhibitors and suggests combination approaches for further clinical studies.
PATIENTS AND METHODS: Formalin-fixed, paraffin-embedded tissue samples of 47 CRCs surgically resected at the Kuala Lumpur Hospital (KLH) between 1999 and 2000 were used. Immunohistochemical staining with monoclonal antibodies against cyclin-D1 and survivin and polyclonal antibodies against Wnt-1 and WISP-1 was performed. Results of immunohistochemistry were analysed for correlation between biomolecules and histopathological data of the patients.
RESULTS: Of the 47 CRCs, 26 (55.3%), 15 (31.9%), 5 (10.6%) and 28 (59.6%) of the tumours exhibited positivity for Wnt-1, WISP-1, cyclin D1 and survivin, respectively. A lower percentage of the 40 apparently normal adjacent tissues were found to be positive for Wnt-1 (7, 17.5%), WISP-1 (+/-5, 12.5%) and survivin (13, 32.5%), but cyclin D1 was not detected in any of them. Interestingly, the total scores of Wnt-1, WISP-1 and survivin were significantly higher in CRC tissues (p=0.001, 0.034 and 0.044, respectively). Using the Spearman rank correlation test, a positive linear relationship was found between total Wnt-1 score with total WISP-1 score (rho=0.319, p=0.003) and total survivin score (rho=0.609, p=or<0.001). The expression of WISP-1 in the CRC tissues was found to be positively correlated with patients older than 60 years old (p=0.011). In addition, nuclear cyclin-D1 expression was found to be associated with poorly differentiated CRC tissues (p<0.001, Table 5) and right-sided CRC tumour (p=0.019, Table 6). Total WISP-1 score was associated with well-differentiated CRC tissues (p=0.029).
CONCLUSIONS: Overexpression and interplay between Wnt-1, WISP-1, survivin and cyclin-D1 may play a role in tumorigenesis, possibly by promoting cell cycle checkpoint progression, accelerating cell growth and inhibiting apoptosis. Our data may provide useful information towards the search for potent therapeutic targets towards the development of novel treatment strategies for CRC.