High 18F-FDG uptake in microscopic peritoneal tumors requires physiologic hypoxia

Abstract

The objective of this study was to examine (18)F-FDG uptake in microscopic tumors grown intraperitoneally in nude mice and to relate this to physiologic hypoxia and glucose transporter-1 (GLUT-1) expression.

Methods: Human colon cancer HT29 and HCT-8 cells were injected intraperitoneally into nude mice to generate disseminated tumors of varying sizes. After overnight fasting, animals, breathing either air or carbogen (95% O(2) + 5% CO(2)), were intravenously administered (18)F-FDG together with the hypoxia marker pimonidazole and cellular proliferation marker bromodeoxyuridine 1 h before sacrifice. Hoechst 33342, a perfusion marker, was administered 1 min before sacrifice. After sacrifice, the intratumoral distribution of (18)F-FDG was assessed by digital autoradiography of frozen tissue sections. Intratumoral distribution was compared with the distributions of pimonidazole, GLUT-1 expression, bromodeoxyuridine, and Hoechst 33342 as visualized by immunofluorescent microscopy.

Results: Small tumors (diameter, <1 mm) had high (18)F-FDG accumulation and were severely hypoxic, with high GLUT-1 expression. Larger tumors (diameter, 1-4 mm) generally had low (18)F-FDG accumulation and were not significantly hypoxic, with low GLUT-1 expression. Carbogen breathing significantly decreased (18)F-FDG accumulation and tumor hypoxia in microscopic tumors but had little effect on GLUT-1 expression.

Conclusion: There was high (18)F-FDG uptake in microscopic tumors that was spatially associated with physiologic hypoxia and high GLUT-1 expression. This enhanced uptake was abrogated by carbogen breathing, indicating that in the absence of physiologic hypoxia, high GLUT-1 expression, by itself, was insufficient to ensure high (18)F-FDG uptake.

Figure 1

¹⁸F-FDG uptake in HT29 peritoneal tumors in air breathing condition. Part of a larger tumor (square) has relatively low levels of ¹⁸F-FDG uptake, PIMO binding, and GLUT-1 expression, with relatively high levels of cell proliferation and blood perfusion. A microscopic tumor (circle) has relatively high FDG uptake, PIMO binding and GLUT-1 with lower cell proliferation and little perfusion. Similar results were seen in 7 animals. Scale bar 1 mm.

Figure 2

Quantitative ¹⁸F-FDG uptake based on a collection of intraperitoneal HT29 tumors derived from 5 tumors (<1mm diameter) and 4 tumors (1–4mm diameter), all from a single air-breathing. ¹⁸F-FDG uptake was significantly higher in smaller tumors than larger ones, P< 0.001.

Figure 3

Comparison of ¹⁸F-FDG uptake with tumor hypoxia, GLUT-1 expression and cellular proliferation in 2 HT29 ascites tumors from an air-breathing animal. Ascites tumors had high ¹⁸F-FDG uptake, pimonidazole binding and GLUT-1 expression with proliferation (BrdUrd) confined to the rim. Scale bar 500 μm.

Figure 4

(A) Comparison of HT29 peritoneal tumors from animals breathing air or carbogen (95% O₂, 5% CO₂). ¹⁸F-FDG uptake and pimonidazole binding were markedly reduced for carbogen breathing whereas GLUT-1 expression was unaffected (see text for details). Scale bar 500 μm. (B) For air breathing conditions, overall ¹⁸F-FDG uptake was higher and a number of “hot spots” were observed. In contrast, carbogen breathing (C) resulted in significantly less¹⁸F- FDG uptake in microscopic tumors. Similar results were seen in 4 carbogen-breathing animals. Scale bars 4 mm. (D) The difference in ¹⁸F-FDG uptake between sub-millimeter HT29 tumors in air-breathing (9 tumors from 2 animals) and carbogen-breathing (11 tumors from 2 animals) animals was significant p<0.001.