Comparison of radiolabeled bromomisonidazole and misonidazole as probes for hypoxic tissue

Misonidazole and related nitroimidazoles may be useful for diagnostic detection of hypoxic cells in tumor, stroke, or myocardial infarct due to binding by metabolic reactions which are limited to cells with low oxygen tension. 4-Bromo-misonidazole has been synthesized from misonidazole. If labeled with a gamma emitting bromine isotope, it may be useful in nuclear imaging of hypoxic tissue. It has been compared to misonidazole in radiolabeled form as a probe for hypoxic cells in an in vitro tumor model, the multi-cellular tumor spheroids. This system is useful for screening nuclear imaging agents. These ''pseudo tumors'' have cell-cell contact, three dimensional growth, and development of central necrosis similar to in vivo tumors, but lack the complications of a vascular system and host physiological influences. EMT-6 tumor cell spheroids with necrotic, hypoxic cores were incubated with 50 μm C-14-bromomisonidazole or C-14-misonidazole. Maximum levels of C-14-bromomisonidazole (approx 5 fold medium levels) were attained by 10 hrs while C-14-misonidazole uptake reached a similar maximum level only after 23 hrs. In autoradiographs, essentially no label from bromomisonidazole or misonidazole accumulated in oxic cells at the periphery of the spheroid while cells presume to be hypoxic but viable adjacent to the necrotic center were heavily labeled at incubation time ≥ 5 hrs. HPLC techniques can separate C-14-misonidazole from C-14-bromomisonidazole and will allow us to determine if bromine remains attached to stably bound C-14 labeled compounds after spheroids have metabolized C-14-bromomisonidazole