LIMITED PENETRATION OF METHOTREXATE INTO HUMAN OSTEO-SARCOMA SPHEROIDS AS A PROPOSED MODEL FOR SOLID TUMOR RESISTANCE TO ADJUVANT CHEMOTHERAPY

Abstract

Oxygen has a limited ability to diffuse into solid tumor masses. The question of the ability of chemotherapy agents to penetrate solid tumor masses has not been evaluated. This clearly would have an impact on the ability of chemotherapy to control microscopic disease during the avascular phase of growth. The ability of methotrexate to penetrate solid tumor masses when grown in 3 dimensions (spheroids) was evaluated. Since methotrexate is used in the clinical management of human osteosarcoma, this drug-tumor combination was chosen. This was done by growing human osteosarcoma cells into spheroids and exposing spheroids of various sizes to 3H-methotrexate. Autoradiographs were obtained from sections through the center of spheroids of various sizes. Methotrexate probably has a limited ability to penetrate into avascular tumor masses when grown in 3 dimensions. This is most evident when the tumor masses are approximately 250 .mu.m and larger in diameter. The degree of penetration of methotrexate was compared to the growth fraction of the tumor, as measured by 3H-thymidine; the growth fraction was much greater than the fraction of cells reached by methotrexate. The limited ability of methotrexate to penetrate solid tumor masses apparently offers an alternative explanation for the limited effectiveness of methotrexate when used as an adjuvant for osteosarcoma. Whether the established biochemical mechanisms for methotrexate resistance are comprehensive explanations for its limited clinical effectiveness is questionable.