BACKGROUND: Despite intensive treatment protocols and recent advances, 
neuroblastomas still account for approximately 15% of all childhood cancer 
deaths. In contrast with adult cancers, p53 pathway inactivation in 
neuroblastomas is rarely caused by p53 mutation but rather by altered MDM2 or 
p14ARF expression. Moreover, neuroblastomas are characterised by high 
proliferation rates, frequently triggered by pRb pathway dysfunction due to 
aberrant expression of cyclin D1, CDK4 or p16INK4a. Simultaneous disturbance of 
these pathways can occur via co-amplification of MDM2 and CDK4 or homozygous 
deletion of CDKN2A, which encodes both p14ARF and p16INK4a.
METHODS AND RESULTS: We examined whether both single and combined inhibition of 
MDM2 and CDK4/6 is effective in reducing neuroblastoma cell viability. In our 
panel of ten cell lines with a spectrum of aberrations in the p53 and pRb 
pathway, idasanutlin and abemaciclib were the most potent MDM2 and CDK4/6 
inhibitors, respectively. No correlation was observed between the genetic 
background and response to the single inhibitors. We confirmed this lack of 
correlation in isogenic systems overexpressing MDM2 and/or CDK4. In addition, 
combined inhibition did not result in synergistic effects. Instead, abemaciclib 
diminished the pro-apoptotic effect of idasanutlin, leading to slightly 
antagonistic effects. In vivo treatment with idasanutlin and abemaciclib led to 
reduced tumour growth compared with single drug treatment, but no synergistic 
response was observed.
CONCLUSION: We conclude that p53 and pRb pathway aberrations cannot be used as 
predictive biomarkers for neuroblastoma sensitivity to MDM2 and/or CDK4/6 
inhibitors. Moreover, we advise to be cautious with combining these inhibitors 
in neuroblastomas.