Influence of melanin on pO2 measurement in vitro and in vivo

Abstract

Abstract. In a previous study an apparent discrepancy was found between the radiobiological hypoxic fraction of tumours and the tumour oxygenation: the lowest percentage of low p O2 values was observed in the most hypoxic tumour, a heavily pigmented melanoma Na11 +. This report describes a similar study with two other less pigmented melanomas. The influence of melanin on p O2 readings was also studied using synthetic melanin and L-tyrosine. Tumour oxygenation was measured using the KIMOC 6650 histograph, apparent p O2 was also measured in the calibration chamber in a buffer containing melanin or L-Tyr at three pHs (6. 5, 7.0, 7.5) and bubbled with three different oxygen concentrations (0. 2, 2.0, 20.9%). The proportion of hypoxic cells, measured by an in vivo / in vitro colony assay, was 58% for Na11 +, 30% for Be11, and 51% for Ma11 tumours. The melanin content (mug/106 cells) was 6.5 (Na11+), 2.0 (Be11), and 4. 3 (Ma11). The percentages of radiobiologically hypoxic cells and low p O2 readings values (< 2 mmHg) were inversely correlated, contrary to what was expected. In buffer, the p O2 values increased significantly with the melanin concentration: the lower the oxygen concentration, the greater was the increase in p O2. The p O2 readings values increased to a lesser extent with L-Tyr concentration. These results indicate that clinical determination of p O2 in melanoma tumours requires careful attention.