# CDK4/6 inhibitors display a class effect in inducing differentiation of neuroblastoma cells

## TL;DR

CDK4/6 inhibitors, when combined with retinoic acid, help aggressive neuroblastoma cells specialize into neurons, suggesting a promising new treatment strategy.

## Contribution

The study demonstrates that CDK4/6 inhibitors have a class effect in inducing neuronal differentiation in neuroblastoma cells.

## Key findings

- Palbociclib, abemaciclib, and ribociclib all enhance retinoic acid-induced differentiation in neuroblastoma cells.
- This effect is observed in both adherent and three-dimensional culture models.
- CDK4/6 inhibitors combined with retinoic acid may offer a new therapeutic strategy for neuroblastoma.

## Abstract

Neuroblastoma is the most common extracranial solid tumour in infants and children, accounting for approximately 15% of paediatric cancer mortality. These tumours are unique in that a subset, namely stage MS, frequently undergo spontaneous regression or differentiation. Differentiation therapy, where cancer cells are re-routed back down their correct developmental pathway, is therefore a promising therapeutic avenue. We have previously shown that the CDK4/6 inhibitor palbociclib induces both decreased proliferation and enhanced neuronal differentiation of neuroblastoma cells
in vitro. When combined with retinoic acid, already used clinically for maintenance therapy, this differentiation is enhanced.

Here, we investigate two additional CDK4/6 inhibitors, abemaciclib and ribociclib, to induce differentiation of the relapsed, high-risk MYCN-amplified neuroblastoma cell line SK-N-BE(2)C, with and without retinoic acid. We culture SK-N-BE(2)C cells in both adherent and three-dimensional culture and monitor proliferation and differentiation using readouts including live-imaging, immunocytochemistry, qRT-PCR and EdU incorporation.

We find the CDK4/6 inhibitors palbociclib, abemaciclib and ribociclib all enhance retinoic acid-induced differentiation in both adherent SK-N-BE(2)C cells and 3D spheroids.

CDK4/6 inhibitors display a class effect in inducing neuronal differentiation together with retinoic acid, both in adherent neuroblastoma cell lines and three-dimensional tumour spheroids. This is an important consideration for potentially developing CDK inhibitor-induced differentiation as a therapy in the clinic.

Neuroblastoma is the most common solid tumour outside of the brain in children and infants. These tumours happen when developing cells in the sympathetic nervous system fail to specialise properly and begin dividing uncontrollably. Rerouting neuroblastoma cells back down their correct pathway is a promising therapeutic strategy that may present fewer long-lasting side-effects than therapies that directly kill cells.

We have previously found that a drug called palbociclib, which interferes with cell division and is used to treat some types of breast cancer, triggers neuroblastoma cells to specialise into neurons. Combining this with retinoic acid, a derivative of vitamin A already used to prevent neuroblastoma tumours regrowing, enhances this effect even more.

In this research article, we wanted to see if this effect is specific to the drug palbociclib. To do this, we test two other drugs which interfere with cell division in the same way as palbociclib. We find that all three drugs, together with retinoic acid, trigger aggressive neuroblastoma cells to specialise into neurons. These drugs therefore have a ‘class effect’. This is important knowledge for developing this therapy from the bench to the clinic, where the drug used must be balanced with factors such as efficacy, availability and cost.