PET imaging of Hsp90 expression in pancreatic cancer using a new ⁶⁴Cu-labeled dimeric Sansalvamide A derivative

Objectives: Heat shock protein (Hsp90) plays a vital role in the progress of malignant disease and elevated Hsp90 expression has been reported in a variety of cancers. In this study, we radiolabeled a dimeric Sansalvamide A derivative (Di-San A1) with ⁶⁴Cu, and evaluated the feasibility of using ⁶⁴Cu-Di-San A1 for PET imaging of Hsp90 expression in a mouse model of pancreatic cancer.

Methods: The Di-San A1 peptide was conjugated with a macrocyclic chelator NOTA to prepare a precursor. Radiolabeling of the precursor with positron emitter ⁶⁴Cu (t_1/2 = 12.7h) was performed in ammonium acetate buffer (pH 5.5) at 45°C for 45 min. The resulting ⁶⁴Cu-Di-San A1 was subsequently subject to in vitro stability, tumor cell uptake, small animal PET, and biodistribution studies.

Results: ⁶⁴Cu-Di-San A1 was successfully prepared with a radiochemical yield >97% and a radiochemical purity >98% (n = 6). ⁶⁴Cu-Di-San A1 is stable in PBS and mouse serum with >90% of parent probe intact after 4 h incubation. The cell uptake study revealed that ⁶⁴Cu-Di-San A1 binds to Hsp90-positive PL45 pancreatic cancer cells, and the binding can be effectively blocked by adding an Hsp90 inhibitor (17AAG). For microPET study, ⁶⁴Cu-Di-San A1 shows good in vivo performance in terms of tumor uptake in nude mice with PL45 tumor xenografts. The Hsp90-specific tumor activity accumulation of ⁶⁴Cu-Di-San A1 was further demonstrated by significant reduction of PL45 tumor uptake with a co-injected blocking dose of 17AAG. The ex vivo PET imaging and biodistribution results were consistent with the quantitative analysis of PET imaging, demonstrating good tumor/muscle ratio (7.39 ± 0.59) of ⁶⁴Cu-Di-San A1 at 4 h post-injection in PL45 tumor mouse xenografts.

Conclusions: A new ⁶⁴Cu-labeled dimeric Sansalvamide A derivative has been successfully developed for PET imaging of Hsp90 expression. ⁶⁴Cu-Di-San A1 shows good and specific PL45 tumor uptake in a mouse model. Further modification of Di-San A1 is underway to optimize the in vivo pharmacokinetics of the tracer, which may provide a non-invasive method to quantitatively characterize Hsp90 expression in pancreatic cancer.