Prediction of antifolate efficacy in a rat sarcoma model

Area of Science:

• Biochemistry
• Pharmacology
• Oncology

Background:

• Thymidylate synthase (TS) is a key enzyme in DNA synthesis and a target for antifolate drugs.
• Predicting patient response to antifolate chemotherapy remains a challenge in cancer treatment.

Purpose of the Study:

• To evaluate a rapid in situ biochemical assay for measuring thymidylate synthase (TS) activity.
• To determine the assay's utility in predicting sensitivity and resistance to folate antagonists: methotrexate (MTX), 10-ethyl-10-deazaaminopterin (10-EDAM), and trimetrexate (TMTX).

Main Methods:

• A methylcholanthrene-induced rat sarcoma model was used, propagated both in vitro and in vivo.
• An in situ biochemical assay measured TS activity in whole cells.
• Cytotoxicity was assessed using a clonogenic assay.
• Antitumor effectiveness was evaluated in rats bearing the sarcoma.

Main Results:

• The in situ TS assay showed excellent correlation with drug-induced cytotoxicity and TS inhibition (p < 0.001) after a 4-hour exposure.
• Tumor cells from in vivo propagated sarcoma were less sensitive to antifolates, but relative drug effectiveness remained: TMTX > 10-EDAM > MTX.
• Continuous drug exposure (10-12 days) achieved cytotoxicity at lower doses.
• In vivo antitumor studies showed trimetrexate (TMTX) was most effective, followed by 10-EDAM, then MTX.

Conclusions:

• The rapid in situ TS assay can effectively predict cellular sensitivity to antifolates.
• This assay, along with the clonogenic assay, shows potential for predicting in vivo antitumor efficacy of antifolates in this rat sarcoma model.