Abstract
1081
Objectives Many cancers upregulate nutrient transport including amino acids which can be targeted for imaging and therapy. System A amino acid transporters can concentrate substrates within cells and provide higher tumor to background ratios and prolonged retention in tumors which are desirable properties for imaging. Since system A is one of the few amino acid transport systems that transport N-methylated amino acids, the objective of this work was to synthesize and radiolabel N-methyl derivatives of the known system A tracer, [123I]IVAIB1 and to assess their tumor imaging properties in a mouse model of high grade glioma.
Methods All compounds studied were synthesized from enantiopure N-boc-α-methyl-L-serine in 4-6 steps. Radiobromination was carried out using peracetic acid, ethanol and [76Br]HBr (aq). Radioiodination reactions were carried out in an ethanol solution with hydrogen peroxide, HCl and Na[123I]I (Nordion). The labeled compounds were fully deprotected and isolated using ion-retardation resin in series with a C-18 sep pak cartridge. Compound purity, specific activity and identity were determined using iTLC and radioHPLC. Me[76Br]BrVAIB and Me[123I]IVAIB was injected i.v. into mice with subcutaneous murine DBT gliomatumors for biodistribution (5 time points, n = 4). Small animal PET studies were carried out with Me[76Br]BrVAIB in mice with intracranial DBT gliomas (n = 4).
Results Me[76Br]BrVAIB and Me[123I]IVAIB were radiolabeled in high yields (70-75%), purity (>95%) and good specific activity (>500 mCi/μg). Me[76Br]BrVAIB showed good uptake in the tumor with the highest uptake at 3.99 + 1.36 %ID/g at 30 min and retention at 3 h post injection with 3.30 + 1.18 %ID/g. Cell uptake studies revealed that Me[76Br]BrVAIB is a good substrate for system A transport with a more than 2 fold decrease in uptake in the presence of MeAIB at 19.8 + 9.8% compared to its desmethyl derivative, [76Br]BrVAIB, 56.1 + 11.6%.2 In vivo PET studies showed good tumor uptake 3 h, p.i. as well as in the clearance organs such as the bladder and kidneys. Tracer retention was still observed in the tumor and bladder after 24 h. Conversely, Me[123I]IVAIB showed lower uptake and retention in the tumor with a maximal uptake of 3.58 + 0.66 %ID/g at 5 min p.i. and only retaining 1.85 + 1.10 %ID/g after 3 h. The compound also was significantly more unstable in vivo with higher than expected thyroid uptakes reaching a max of 44.49 + 20.57 %ID/g at 3 h p.i. and still retaining 29.79 + 9.79 %ID/g after 24 h. Cell uptake studies unexpectedly showed no substantial transport of Me[123I]IVAIB by system A.
Conclusions Me[76Br]BrVAIB and Me[123I]IVAIB were easily synthesized and radiolabeled in good yields. Me[76Br]BrVAIB is a system A substrate in vitro with high tumor uptake and retention up to 24 h after injection. While Me[123I]IVAIB was taken up in the tumors, it proved to be unstable in vivo with rapid deiodination and does not appear to be a good system A substrate.