Transport of 5-fluorouracil into primary human cells

[Speaker] Kathryn E Burns:1
[Co-author] Daniel Allright:1, Nuala A Helsby:1
1:Molecular Medicine and Pathology, University of Auckland, New Zealand

Background. 5-fluorouracil (5-FU) is an effective chemotherapeutic agent used widely in the treatment of gastrointestinal and breast cancers. However its use is associated with severe (≥Grade 3) normal tissue toxicities, including mucositis. Although deleterious variants in DPYD have been associated with the development of these side effects, the sensitivity of DPYD genotyping tests is poor (<25%) and for many individuals the underlying cause of these adverse events is unknown. In addition, the transport processes mediate 5-FU influx and efflux in normal tissues are poorly understood.
Methods. We have developed a functional assay to characterise 5-FU transport into primary human cells. Uptake of [2-14C] 5-FU (μM, 37 °C) into a) buccal mucosal cells (BMCs) and b) peripheral blood mononuclear cells (PBMCs) was assessed ex vivo in a cohort of healthy volunteers (n=25). BMC uptake was retested weekly and/or monthly for up to five months in a subset these participants (n=16) to determine intra-individual variability in 5-FU uptake.
Results. Uptake of 5-FU into BMCs was temperature dependent and saturable. Significant (p<0.0001) inter-individual variability was observed at the initial test, with 5-FU uptake into BMCs ranging from 0.5 to 13.4 pmol.min-1.105 live cells-1 across 25 healthy participants. Repeated phenotype testing demonstrated intra-individual variability in BMC 5-FU uptake in some participants (e.g. 5FU002, range 2.0-56.4 pmol.min-1.105 live cells-1) but not in others (e.g. 5FU015, range 0.0-1.3 pmol.min-1.105 live cells-1. In contrast to the BMC data, no uptake of 5-FU into PBMCs was detected.
Conclusions. 5-FU is highly muco-toxic, with the prevalence of mucositis reported to be between 20-50% following treatment with chemotherapeutic regimens that include this drug. To date there has been little focus on the mechanisms that mediate 5-FU accumulation into these cells. Our data show that transport of 5-FU into primary human BMCs, unlike PBMCs, is variable both between and within individuals. The impact of this process on the development of 5-FU induced normal tissue toxicities is worthy of further investigation.
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