Defining the mechanism of resistance to gene-directed enzyme/prodrug therapy mediated by MSC in colon cancer model

Introduction

Mesenchymal stromal cell-mediated gene-directed enzyme/prodrug therapy has been confirmed to be a promising therapeutic approach in many preclinical studies. MSC are able to convert nontoxic prodrug to toxic chemotherapeutic in the site of the tumor with minimal negative impact on healthy tissues. Much work remains ahead to find out to what extent genetically modified MSC are able to target aggressive chemoresistant subpopulations of tumor cells, as well as cancer stem cells.

Materials/methods

Chemoresistant derivative of colorectal adenocarcinoma cells was cocultured with engineered mesenchymal stromal cells (MSC) expressing yeast cytosine deaminase (CD) or yeast fusion cytosine deaminase::uracil p h o s p h o r i b o s y l t r a n s f e r a s e (CD::UPRT) to determine the cytotoxic effect of therapeutic MSC on chemoresistant tumor cells. Quantitative PCR was conducted to determine the expression of enzymes involved in the 5-FU metabolism.

Results and conclusions

Gene-directed enzyme/prodrug therapy (GDEPT) mediated by MSC is based on the bystander effect when prodrug (5-Fluorocytosine, 5-FC) is converted to toxic metabolites by engineered MSC. Toxic intermediates pass the membrane and induce the apoptosis of surrounding tumor cells. Chemoresistant cells showed decreased response to MSCmediated GDEPT in comparison to their parental counterparts. The resistance has increased with the number of passages in vitro and has been accompanied with increased activity of aldehyde dehydrogenase. We observed significant changes in the expression of enzymes involved in metabolism of 5-FC. Pharmacologic inhibition of thymidine phosphorylase and thymidylate synthase led to alterations in the resistance to chemotherapy. Direct expression of prodrug-converting enzyme in chemoresistant tumor cells did not sensitize them to 5-FC.

We demonstrated that resistance to 5-FU can affect the efficacy of GDEPT mediated by CD::UPRT-expressing MSC. Although the gene therapy is still an experimental treatment approach, first clinical studies employing genetically engineered MSC were initiated. We assume that the possibility of decreased efficacy of the therapeutic system at 5-FU treated patients should be taken into consideration at definition of clinical studies‘ inclusion criteria.

Financial support: APVV-0052- 12 and APVV-0230-11; VEGA 2/0171/13, 2/0130/13 and 2/0087/15.

This work was partially supported by the WAC and RFL programs funded by the Slovak Cancer Research Foundation.