Radiotoxicity of some iodine-123, iodine-125 and iodine-131-labeled compounds in mouse testes: implications for radiopharmaceutical design.

In this work, spermhead survival in mouse testis was used to investigate the radiotoxicity of several intratesticularly localized radioiodinated pharmaceuticals. Radioiodines that decay by electron capture and/or internal conversion (123I, 125I) as well as by beta- decay (131I) were coupled to pharmaceuticals that selectively localize in different cell compartments. Dose response curves yield D37 values of 62 cGy, 75 cGy, 61 cGy and 7.7 cGy for 123IMP (N-isopropyl-p-iodoamphetamine), 131IdU (iododeoxyuridine), H131IPDM (N,N,N'-trimethyl-N'-(2-hydroxyl-3-methyl-5-iodobenzyl)-1,3-propanediami ne) and 125IdC (iododeoxycytidine), respectively. At 37% survival, the relative biological effectiveness (RBE) of these radiochemicals, when compared to the pure gamma-emitting radiochemical 7Be-chloride (D37 = 65 cGy), are 1.0, 0.89, 1.1 and 8.4, respectively. Intratesticular 7Be, with an effective half-life of 430 hr in the organ, was used as the source of reference radiation to determine the RBE values because it solely emits 477 keV gamma rays, and the dose to the testis is delivered chronically, as in the case of the other radiocompounds. Subcellular distribution studies show that all of the cellular activity is localized in the cytoplasm in the cases of 123IMP and H131IPDM, while virtually all of 131IdU and 125IdC were bound to DNA in the cell nucleus. In agreement with our earlier in vivo studies, these data show that subcellular distribution plays a key role in the radiotoxicity of Auger electron emitters such as 123I and 125I, and has no role for beta emitters such as 131I. These findings may have implications in the design of radiopharmaceuticals for both diagnosis (localize Auger emitter in cytoplasm of cell) and therapy (localize Auger emitter in cell nucleus).