Radiotracer-based method for determining water solubility of highly insoluble compounds

Evaluation of Chemical, Physical, and Biologic Properties of Tumor-Targeting Radioiodinated Quinazolinone Derivative

Our group is developing a novel technology, enzyme-mediated cancer imaging and therapy (EMCIT), that aims to entrap radioiodinated compounds within solid tumors for noninvasive tumor detection and therapy. In this approach, a water-soluble, radioiodinated prodrug is hydrolyzed in vivo to a highly water-insoluble compound by an enzyme overexpressed extracellularly by tumor cells. We have synthesized and characterized the water-soluble prodrug, 2-(2'-phosphoryloxyphenyl)-6-[(125)I]iodo-4-(3H)-quinazolinone [(125)I]5, which is readily hydrolyzed by alkaline phosphatase, an enzyme expressed by many tumor cell lines, to a water-insoluble drug, 2-(2'-hydroxyphenyl)-6-[(125)I]iodo-4-(3H)-quinazolinone [(125)I]1. In the course of our study, we discovered that ammonium 2-(2'-phosphoryloxyphenyl)-6-tributylstannyl-4-(3H)-quinazolinone, an intermediate in the radioiodination of the prodrug, exists as two isomers (3 and 4) whose radioiodination leads, respectively, to [(125)I]6 and [(125)I]5. These prodrugs have different in vitro and in vivo biologic activities. Compound 6 is not hydrolyzed by alkaline phosphatase (ALP), whereas 5 is highly soluble (mg/mL) in aqueous solution and is rapidly dephosphorylated in the presence of ALP to 1, a water-insoluble molecule (ng/mL). Mouse biodistribution studies indicate that [(125)I]6 has high uptake in kidney and liver and [(125)I]5 has very low uptake in all normal organs. Compounds 3 and 6 are converted, respectively, to 4 and 5 after incubation in DMSO. The stability of 5 in human serum is high. The minimum ALP concentration needed to hydrolyze 5 is much greater than the ALP level in the blood of patients with cancer, and the latter should not affect the pharmacokinetics of the compound. Incubation of 5 with viable human and mouse tumor-cell lines--but not with normal human cells and mouse tissues--leads to its hydrolysis and the formation of large crystals of 1. We expect that 5 will also be hydrolyzed in vivo by tumor cells that express phosphatase activity extracellularly and anticipate the specific precipitation of radioiodinated 1 within tumor cell clusters. This should lead to high tumor-to-normal-tissue ratios and enable imaging (SPECT/PET) and radionuclide therapy of solid tumors.

Molecular-Docking-Guided Design, Synthesis, and Biologic Evaluation of Radioiodinated Quinazolinone Prodrugs

Enzyme-mediated cancer imaging and therapy (EMCIT) is a novel approach in which radioactive water-soluble molecules are precipitated in vivo following their hydrolysis by extracellular enzymes overexpressed by cancer cells. AutoDock 3.0 was used to model the interaction-binding between a series of iodinated quinazolinone derivatives and human placental alkaline phosphatase (PLAP, crystal structure in the Protein Data Bank) and to assess the effects of structural modification of the derivatives. Ammonium 2-(2',4'-diphosphoryloxyphenyl)-6-iodo-4-(3H)-quinazolinone (IQ2-P,4-P), having the most favorable calculated inhibition constant, was synthesized and characterized. Concentration-dependent, PLAP-mediated conversion of IQ2-P,4-P (4)/125IQ2-P,4-P (6) to water-insoluble 2-(2',4'-dihydroxyphenyl)-6-[127I/125I]iodo-4-(3H)-quinazolinone (127IQ2-OH,4-OH (2)/125IQ2-OH,4-OH (7)) was observed in solution. Autoradiography indicated that 6 is hydrolyzed by human cancer cells and the resulting 7 precipitates on exterior cell surfaces. Biodistribution studies in mice demonstrated that 6 is minimally retained by normal tissues. The findings support the validity of the EMCIT approach.