Many patients with retinoblastoma have a genetic predisposition to cancer and external beam radiation therapy and alkylating agent chemotherapy may increase their risk of secondary malignancy. Identification of effective chemotherapy agents for retinoblastoma that are not associated with an elevated risk of secondary malignancy would be beneficial.
Twenty-six specimens of fresh retinoblastoma tumor cells were studied in vitro with a PT430 competitive displacement assay. Differential displacement of the PT430 by methotrexate and not trimetrexate was considered indicative of a defect in reduced folate carrier (RFC)-mediated transport. Elevations in the accumulation of PT430 were considered indicative of dihydrofolate reductase (DHFR) amplification.
In 9 of the 26 (35%) samples, displacement by methotrexate was less than half the displacement by trimetrexate indicative of a defect in the RFC. In 5 of the 26 (19%) samples, trimetrexate did not displace the PT430. In 7 of 26 (27%) samples, the peak PT430 accumulation was suggestive of DHFR overexpression. Overall 9 of 26 (35%) samples had no evidence of a transport defect or DHFR overexpression and would be anticipated to be potentially sensitive to methotrexate. In 15 of the 26 (58%), no defects existed in trimetrexate displacement or DHFR overexpression and would be anticipated to be potentially sensitive to trimetrexate.
These results would support consideration of a phase II study to determine the effectiveness of trimetrexate for recurrent intra-ocular retinoblastoma.
"In two of the 69 samples, PT430 displacement by methotrexate was greater than displacement by trimetrexate which may be related to P-glycoprotein expression. Nine samples had elevated peak PT430 accumulation signifying possible DHFR overexpression  . All 50 biopsy samples exhibited less than 20% methotrexate displacement of PT430 (range = 0% to 19.3%; mean = 3.6%) with 16 samples displaying 0% displacement by methotrexate. "
[Show abstract][Hide abstract] ABSTRACT: Osteosarcoma does not respond well to conventional dose methotrexate but does respond to high-dose methotrexate. Previous work has indicated that this resistance may be due to impaired transport of methotrexate across the cell membrane. In this study, the PT430 competitive displacement assay was adapted to evaluate methotrexate transport in 69 high-grade osteosarcoma tumor samples. All samples studied were shown to have relatively impaired methotrexate transport by PT430 assay. Ninety-nine percent of the samples had less than 20% PT430 displacement by methotrexate. Eighty-eight percent exhibited displacement by methotrexate at less than 50% of the displacement by trimetrexate. The high frequency of impaired transport suggests the presence of decreased functionality of the reduced folate carrier protein. The overwhelming presence of impaired transport may explain why methotrexate needs to be given in high doses to be effective in osteosarcoma therapy and suggests that reduced folate carrier-independent antifolates should be explored.
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