Effect of breathing carbogen on the oxygen tension of murine and human tumours measured using an Eppendorf pO2 histograph

The Impact of Complement Activation on Tumor Oxygenation During Photodynamic Therapy

The response to photodynamic therapy (PDT) mediated by photosensitizer Photofrin was examined with Lewis lung carcinomas growing in either complement-proficient C57BL/6 (B6) or complement-deficient complement C3 knockout (C3KO) mice. The results reveal that Photofrin-PDT was more effective in attaining cures of tumors in C3KO than in B6 hosts. Colony-forming ability of cells from tumors excised immediately after Photofrin-PDT confirmed that the direct cell killing effect was more pronounced in C3KO than in B6 hosts. In contrast, PDT mediated by photosensitizer benzoporphyrin derivative (BPD) produced higher cure rates of tumors in B6 hosts than those in C3KO hosts. Determination of tumor C3 levels by ELISA showed that Photofrin-PDT induced markedly more pronounced complement activation than BPD-PDT. Measurements of tumor oxygen tension immediately after PDT by Eppendorf pO2 histograph showed that Photofrin-PDT induced a marked decline in the oxygenation of tumors growing in B6 mice that was much less pronounced in C3KO hosts. With BPD-PDT the oxygen tensions in tumors in B6 and C3KO hosts decreased to a similar extent. This study indicates that complement activation in PDT-treated tumors that varies with different photosensitizers is an important determinant of tumor oxygen limitation effects directly associated with photodynamic action.

Carbogen breathing after irradiation enhances the effectiveness of tirapazamine in SiHa tumors but not SCCVII tumors in mice

The penetration of anticancer agents into tumor tissue has recently attracted considerable attention. This study examines the effect of carbogen breathing on the antitumor activity of tirapazamine combined with radiation. Our hypothesis is based on the observation that the diffusion of tirapazamine through tissue is dependent on oxygen tension. We postulated that carbogen breathing might enhance the ability of tirapazamine to diffuse to hypoxic cells located distal to functional blood vessels in tumors. We first determined that carbogen breathing caused no significant change in the pharmacokinetics of tirapazamine, suggesting that any effect of carbogen breathing on the activity of tirapazamine is not attributable to modulation of pharmacokinetics. Cell survival in SCCVII and SiHa tumors after 10 Gy X rays alone was similar. However, when tirapazamine was administered 30 min after radiation treatment under air-breathing conditions, cell killing was greater in SCCVII tumors compared to SiHa tumors. Carbogen breathing during the exposure to tirapazamine did not change the cell survival in SCCVII tumors, but it enhanced cell killing in the SiHa tumors. Interestingly, carbogen breathing during radiation treatment produced greater cell killing in the SiHa tumors than in the SCCVII tumors. The vascular architecture and type of hypoxia in the two tumors probably underlie the differences in the responses of the two tumors. These findings suggest that the effectiveness of tirapazamine and other hypoxic cytotoxins may be dependent on tumor type.

The modification of human tumour blood flow using pentoxifylline, nicotinamide and carbogen

AIM

To assess the effect of combining oral nicotinamide, oral pentoxifylline and carbogen gas (2% CO2, 98% O2) breathing on human tumour red cell flux.

METHODS AND MATERIALS

Microregional red blood cell flux was measured in accessible tumour nodules using laser Doppler microprobes in 11 patients with histologically proven malignancy. Patients received single oral doses of nicotinamide 40 mgkg-1 and pentoxifylline 1200 mg 2h before a 10-min period of carbogen gas breathing, corresponding to peak plasma concentrations of these drugs. Red cell flux in up to six microregions in each tumour was measured for 30 min, recording pre-, during and post-carbogen breathing for 10 min each.

RESULTS

Data from ten of the 11 patients could be assessed. The red cell flux in 48 microregions was analysed and the mean red cell flux was calculated. A mean relative increase in red cell flux of 1.18 (+/-0.09, 95% confidence interval (CI)) was observed after 6 min of carbogen breathing, 2h after the administration of nicotinamide and pentoxifylline. This compares to relative increases of 1.4 (+/-0.39, 95%CI) after nicotinamide with carbogen and 1.15 (+/-0.10, 95%CI) after pentoxifylline with carbogen. These differences are not statistically significant (P>0.05). The increased red cell flux persisted after the cessation of carbogen gas breathing.

CONCLUSIONS

A combination of pentoxifylline, nicotinamide and carbogen produces an increase in human tumour red cell flux, similar to that observed when each of the drugs are used alone with carbogen breathing.