Biochemical modulation of 5-fluorouacil through dihydropyrimidine dehydrogenase inhibition: a Southwest Oncology Group phase II trial of eniluracil and 5-fluorouracil in advanced resistant colorectal cancer

Purpose. To investigate thehypothesis that a systemic agent designedto inhibit dihydropyrimidine dehydrogenase(DPD), the first enzyme in thefluoropyrimidine degradative pathway, couldimprove the effective amount of5-fluorouracil (5-FU) delivered to a tumorresulting in enhanced response.Patients and methods. Eligibility includedcytologically or pathologically verifieddiagnosis of colorectal cancer thatrecurred during or within 12 months ofcompletion of adjuvant therapy,representing patients generally consideredresistant to fluorinated pyrimidinetherapy. Stratification was into twocohorts: recurrence while receivingadjuvant therapy, and relapse within 12months of completing adjuvant therapy.Treatment consisted of 28 days of oraltherapy every five weeks with eniluraciland 5-FU administered in a 10:1 ratio. Thedaily dose of eniluracil was 10 mg/m²with 5-FU 1 mg/m², divided into twodoses.Results. Twenty-five patientsare evaluable for response: 9 relapsedduring therapy and 16 relapsed within oneyear of adjuvant therapy. In the firstgroup, there was one partial response (9%;95% CI 0–41%); in the second cohort therewas one confirmed complete response (5%;95% CI 0–23%) and one unconfirmed partialresponse, for an overall response rate of10%.Conclusions. This regimen lackssignificant activity in this targetpopulation. Pre-treatment intratumoral DPDexpression was not assessed, therefore themechanism of fluorinated pyrimidineresistance cannot be specificallyattributed to elevated DPD levels.Attempting restoration of chemotherapysensitivity through blockade of enzymes orsignal transduction molecules responsiblefor resistance is rational, provided thattumor target expression is the basis fortrial entry.