Copper-64-labeled anti- bcl-2 PNA-peptide conjugates selectively localize to bcl-2 -positive tumors in mouse models of B-cell lymphoma

Abstract Introduction The bcl-2 gene is overexpressed in non-Hodgkin's lymphoma (NHL). We have reported micro-SPECT/CT imaging of Mec-1 human lymphoma xenografts in SCID mice, using [111 In]DOTA-anti- bcl-2 -PNA-Tyr3 -octreotate. In order to reduce normal organ accumulation and improve imaging contrast, modified monomers with neutral hydrophilic (serine, TS ) or negatively charged (aspartic acid, TD ) residues were synthesized as substitutes for glycine at T14 in the PNA sequence. The parent and modified PNA-peptide conjugates were labeled with64 Cu and evaluated in biodistribution studies and high resolution PET/CT imaging of SCID mice bearing bcl-2 -postive Mec-1 xenografts as well as bcl-2 -negative Ramos xenografts. Methods Mice were administered the64 Cu-labeled conjugates for biodistribution and imaging studies. Biodistributions were obtained from 1 to 48 h post-injection. Mice were imaged from 1 to 48 h post-injection. Results The parent glycine conjugate and two modified conjugates all showed selective tumor uptake in Mec-1 xenografts. The liver uptake of the serine conjugate was significantly reduced compared to the two other PNA conjugates. Its kidney uptake was highest of the three at 47.1% ID/g at 1 h and dropped to 20.6% ID/g at 24 h. [Copper-64]DOTA-anti- bcl-2 -TS -PNA-Tyr3 -octreotate showed tumor uptake of 1.38% ID/g at 1 h and 1.06% ID/g at 24 h. The tumor-to-blood ratio was increased by factor of 2 from 1 h to 24 h. This compound detected Mec-1 tumors by micro-PET/CT as early as 1 h post-injection and at time points out to 48 h. However, the negative control Ramos tumor could not be detected. Conclusions These64 Cu-labeled, amino acid-modified PNA conjugates showed selective tumor targeting in vivo, and tumor xenografts were detected by micro-PET/CT as early as 1 h post-injection, suggesting that bcl-2 expression at the mRNA level can detected by PET in mouse models of NHL. Advances in knowledge and implications for patient care Down-regulating bcl-2 , an anti-apoptotic proto-oncogene, is a mechanism to reverse chemotherapy resistance in humans with NHL. Thus, bcl-2 overexpression might be considered a new independent prognostic parameter in NHL, aiding in the identification of patients at risk for treatment failure. We have developed [64 Cu]DOTA-anti- bcl-2 -PNA-Tyr3 -octreotate conjugates for targeted antisense PET imaging. Our preclinical studies identified an effective combination of antisense and radionuclide imaging, with the goal