In silico design, synthesis, and biological evaluation of radioiodinated quinazolinone derivatives for alkaline phosphatase-mediated cancer diagnosis and therapy.

As part of the development of enzyme-mediated cancer imaging and therapy, a novel technology to entrap water-insoluble radioactive molecules within solid tumors, we show that a water-soluble, radioactive quinazolinone prodrug, ammonium 2-(2'-phosphoryloxyphenyl)-6-[125I]iodo-4-(3H)-quinazolinone (125IQ(2-P)), is hydrolyzed by alkaline phosphatase to a water-insoluble, radiolabeled drug, 2-(2'-hydroxyphenyl)-6-[125I]iodo-4-(3H)-quinazolinone (125IQ(2-OH)). Biodistribution data suggest the existence of two isoforms of the prodrug (IQ(2-P(I)) and IQ(2-P)), and this has been confirmed by their synthesis and characterization. Structural differences of the two isoforms have been examined using in silico molecular modeling techniques and docking methods to describe the interaction/binding between the isoforms and human placental alkaline phosphatase (PLAP), a tumor cell, membrane-associated, hydrolytic enzyme whose structure is known by X-ray crystallographic determination. Docking data show that IQ(2-P), but not IQ(2-P(I)), fits the active binding site of PLAP favorably and interacts with the catalytic amino acid Ser(92), which plays an important role in the hydrolytic process. The binding free energies (DeltaG(binding)) of the isoforms to PLAP predict that IQ(2-P) will be the better substrate for PLAP. The in vitro incubation of the isoforms with PLAP leads to the rapid hydrolysis of IQ(2-P) only and confirms the in silico expectations. Fluorescence microscopy shows that in vitro incubation of IQ(2-P) with mouse and human tumor cells causes the extracellular, alkaline phosphatase-mediated hydrolysis of the molecule and precipitation of fluorescent crystals of IQ(2-OH). No hydrolysis is seen in the presence of normal mouse and human cells. Furthermore, the intratumoral injection of 125IQ(2-P) into alkaline phosphatase-expressing solid human tumors grown s.c. in nude rats results in efficient hydrolysis of the compound and retention of approximately 70% of the injected radioactivity, whereas similar injection into normal tissues (e.g., muscle) does not produce any measurable hydrolysis (approximately 1%) or retention of radioactivity at the injected site. These studies support the enzyme-mediated cancer imaging and therapy technology and show the potential of such quinazolinone derivatives in the in vivo radiodetection (123I/124I) and therapy (131I) of solid tumors.