⁶⁴CuATSM pharmacokinetics in FaDu tumors and correlation to microscopic hypoxia

Objectives To determine (i) the ⁶⁴CuATSM pharmacokinetics by dynamic PET imaging and (ii) the microdistribution by digital autoradiography in a head & neck tumor model FaDu.

Methods Eight FaDu tumor xenografts of 20 ± 4 mm were grown in nude rats. PET imaging determined venous input functions, and time-activity in tissue. Influx constant (K_i) and distribution volume (V_d) were calculated for tumors and healthy muscle. ⁶⁴CuATSM autoradiographs at early (1 h) and late (>18 h) times post-injection were compared to immunohistochemical markers for hypoxia (Pimonidazole) and perfusion (Hoechst 33342).

Results Estimated K_i was 3.1x10^-2 min^-1 for tumor and 3.9x10^-3 min^-1 for muscle, a 12 fold difference. The tumor-to-muscle ratio of V_d was 4.6 ± 0.2 (p = 0.0002), while effective washout rates were indistinguishable (p = 0.06). Mean PET signal (PET_mean) in muscle stayed constant (p >> 0.01) beyond 20 min post-injection. However, PET_mean in tumors only reached pseudo steady state. At 20 min, tumor PET_mean were 4.7 ± 0.2 times muscle, increasing to 5.2 ± 0.4 at 80 min, and 12.2 ± 0.8 by 18 h. Yet, the spatial distributions between PET image voxels appeared unchanged (Pearson’s r > 0.6). No significant trends emerged between ⁶⁴CuATSM and Pimonidazole staining intensity. Bimodal behavior was observed at early times. Linear trends were found in late images, but varied from positive to negative correlation even among multiple cross-sections of the same tumor. ⁶⁴CuATSM was co-distributed with Hoechst 33342 at 1 h.

Conclusions ⁶⁴CuATSM PET distributions were unchanged after 20 min, yet tumor-to-muscle ratios continually increased even at 18 h. The oxygen partial pressures at which ⁶⁴CuATSM and Pimonidazole are reduced and bound in cells are theorized to be distinct and separable. However, microscopic imaging has shown random relationships between distributions of Pimonidazole and accumulation of ⁶⁴CuATSM in this head & neck tumor model. ⁶⁴CuATSM uptake demonstrated clear tumor targeting, but was unable to discern between different microenvironmental sub-regions.

Research Support NIH grant P01 CA11567