[(18)F]FMISO and [(18)F]FDG PET imaging in soft tissue sarcomas: correlation of hypoxia, metabolism and VEGF expression.

Hypoxia imparts resistance to radiotherapy and chemotherapy and also promotes a variety of changes in tumor biology through inducible promoters. The purpose of this study was to evaluate the use of positron emission tomography (PET) imaging with fluorine-18 fluoromisonidazole (FMISO) in soft tissue sarcomas (STS) as a measure of hypoxia and to compare the results with those obtained using [(18)F]fluorodeoxyglucose (FDG) and other known biologic correlates. FDG evaluates energy metabolism in tumors while FMISO uptake is proportional to tissue hypoxia. FMISO uptake was compared with FDG uptake. Vascular endothelial growth factor (VEGF) expression was also compared with FMISO uptake. Nineteen patients with STS underwent PET scanning with quantitative determination of FMISO and FDG uptake prior to therapy (neo-adjuvant chemotherapy or surgery alone). Ten patients receiving neo-adjuvant chemotherapy were also imaged after chemotherapy but prior to surgical resection. Standardized uptake value (SUV) was used to describe FDG uptake; regional tissue to blood ratio (>or=1.2 was considered significant) was used for FMISO uptake. Significant hypoxia was found in 76% of tumors imaged prior to therapy. No correlation was identified between pretherapy hypoxic volume (HV) and tumor grade ( r=0.15) or tumor volume ( r=0.03). The correlation of HV with VEGF expression was 0.39. Individual tumors showed marked heterogeneity in regional VEGF expression. The mean pixel-by-pixel correlation between FMISO and FDG uptake was 0.49 (range 0.09-0.79) pretreatment and 0.32 (range -0.46-0.72) after treatment. Most tumors showed evidence of reduced uptake of both FMISO and FDG following chemotherapy. FMISO PET demonstrates areas of significant and heterogeneous hypoxia in soft tissue sarcomas. The significant discrepancy between FDG and FMISO uptake seen in this study indicates that regional hypoxia and glucose metabolism do not always correlate. Similarly, we did not find any relationship between the hypoxic volume and the tumor volume or VEGF expression. Identification of hypoxia and development of a more complete biologic profile of STS will serve to guide more rational, individualized cancer treatment approaches.

Platelet-derived growth factor C (PDGF-C), a novel growth factor that binds to PDGF alpha and beta receptor.

We have characterized platelet-derived growth factor (PDGF) C, a novel growth factor belonging to the PDGF family. PDGF-C is a multidomain protein with the N-terminal region homologous to the extracellular CUB domain of neuropilin-1, and the C-terminal region consists of a growth factor domain (GFD) with homology to vascular endothelial growth factor (25%) and PDGF A-chain (23%). A serum-sensitive cleavage site between the two domains allows release of the GFD from the CUB domain. Competition binding and immunoprecipitation studies on cells bearing both PDGF alpha and beta receptors reveal a high affinity binding of recombinant GFD (PDGF-CC) to PDGF receptor-alpha homodimers and PDGF receptor-alpha/beta heterodimers. PDGF-CC exhibits greater mitogenic potency than PDGF-AA and comparable or greater mitogenic activity than PDGF-AB and PDGF-BB on several mesenchymal cell types. Analysis of PDGF-CC in vivo in a diabetic mouse model of delayed wound healing showed that PDGF-CC significantly enhanced repair of a full-thickness skin excision. Together, these studies describe a third member of the PDGF family (PDGF-C) as a potent mitogen for cells of mesenchymal origin in in vitro and in vivo systems with a binding pattern similar to PDGF-AB.

Determining hypoxic fraction in a rat glioma by uptake of radiolabeled fluoromisonidazole.

The usefulness of radiolabeled nitroimidazoles for measuring hypoxia will be clarified by defining the relationship between tracer uptake and radiobiologically hypoxic fraction. We determined the radiobiologically hypoxic fraction from radiation response data in 36B10 rat gliomas using the paired cell survival curve technique and compared the values to the radiobiologically hypoxic fraction inferred from mathematical modeling of time-activity data acquired by PET imaging of [(18)F]FMISO uptake. Rats breathed either air or 10% oxygen during imaging, and timed blood samples were taken. The uptake of [(3)H]FMISO by 36B10 cells in vitro provided cellular binding characteristics of this radiopharmaceutical as a function of oxygen concentration. The radiobiologically hypoxic fraction determined for tumors in air-breathing rats using the paired survival curve technique was 6.1% (95% CL = 4.3- 8.6%), which agreed well with that determined by modeling FMISO time-activity data (7. 4%; 95% CL = 2.5-17.3%). These results are consistent with the agreement between the two techniques for measuring radiobiologically hypoxic fraction in Chinese hamster V79 cell spheroids. In contrast, the FMISO-derived radiobiologically hypoxic fraction in rats breathing 10% oxygen was 13.1% (95% CL 7.9-8.3%), much lower than the radiobiologically hypoxic fraction of 43% determined from the radiation response data. This discrepancy may be due to the failure of FMISO to identify hypoxic cells residing at or above an oxygen level of 2-3 mmHg that will still confer substantial protection against radiation. The presence of transiently hypoxic cells in rats breathing reduced oxygen may also be under-reported by nitroimidazole binding, which is strongly dependent on time and concentration.

Characterization of [18F]fluoroetanidazole, a new radiopharmaceutical for detecting tumor hypoxia.

UNLABELLED: Fluorinated derivatives of etanidazole are being explored as probes for tumor hypoxia. Our research group has synthesized [18F]fluoroetanidazole (FETA) and now reports the oxygen dependency of binding to cells in vitro, the biodistribution of the tracer in tumor-bearing mice and the analysis of metabolites in their plasma and urine. METHODS: Four cultured rodent cell lines (V79, 36B10, EMT6 and RIF1) were incubated with [18F]FETA for various times under graded O2 concentrations. We also compared the biodistributions of [18F]FETA and [18F]fluoromisonidazole (FMISO) at 2 and 4 h postinjection in C3H mice bearing KHTn tumors (130-430 mg). Reverse-phase high-performance liquid chromatography was used to distinguish metabolites from parent drugs in urine and plasma of mice injected with [18F]FETA or [18F]FMISO. RESULTS: In cells labeled in vitro, O2 levels of 600-1300 ppm inhibited binding by 50% relative to uptake under anoxic conditions (<10 ppm). These inhibitory values are not statistically different from those reported for [18F]FMISO in the same cell lines (700-1500 ppm). In the biodistribution studies, uptake in heart, intestine, kidney and tumor was similar for both tracers 4 h after injection, whereas retention of [18F]FETA in liver and lung was significantly lower. Less uptake of [18F]FETA in liver suggests that this nitroimidazole is metabolized less than [18F]FMISO. The brain-to-blood ratios indicate that [18F]FETA readily crosses the blood-brain barrier. High-performance liquid chromatography of urine demonstrated that 10% of [18F]FETA-derived activity was in metabolites at 2 h postinjection, with 15% in metabolites by 4 h; comparable values for [18F]FMISO were 36% and 57%, respectively. CONCLUSION: We conclude from these data that [18F]FETA holds promise as a new hypoxia tracer in patients, having oxygen dependency of binding similar to [18F]FMISO in vitro and displaying less retention in liver and fewer metabolites in vivo.