Several alpha particle emitting radioisotopes have been studied for use in radioimmunotherapy. Ac-225 has the potential advantages of a relatively long half life of 10 days, and a yield of 4 alpha emissions in its decay chain with a total energy release of approximately 28 MeV. A new, 12 coordination site chelating ligand, HEHA, has been chemically modified for coupling to targeting proteins without loss of chelating ability. HEHA was coupled with MAb 201B which binds to thrombomodulin and accumulates efficiently in murine lung. Ac-225 was bound to the HEHA-MAb 201B conjugate and injected into BALB/c mice bearing lung tumor colonies of EMT-6 mammary carcinoma. Biodistribution data at 1 and 4 h postinjection indicated that, as expected, 225Ac was delivered to lung efficiently (> 300% ID/g). The 225Ac was slowly released from the lung with an initial t1/2 = 49 h, and the released 225Ac accumulated in the liver. Injection of free HEHA was only partially successful in scavenging free 225Ac. In addition to the slow release of 225Ac from the chelate, data indicated that decay daughters of 225Ac were also released from the lung. Immediately after organ harvest, the level of 213Bi, the third alpha-decay daughter, was found to be deficient in the lungs and to be in excess in the kidney, relative to equilibrium values. Injected doses of 225Ac MAb 201B of 1.0 microCi, delivering a minimum calculated absorbed dose of about 6 Gy to the lungs, was effective in killing lung tumors, but also proved acutely radiotoxic. Animals treated with 1.0 microCi or more of the 225Ac radioconjugate died of a wasting syndrome within days with a dose dependent relationship. We conclude that the potential for 225Ac as a radioimmunotherapeutic agent is compromised not only by the slow release of 225Ac from the HEHA chelator, but most importantly by the radiotoxicity associated with decay daughter radioisotopes released from the target organ.

Second generation monoclonal antibodies to the alpha 6 beta 4 subunits of human integrins have been prepared. MAbs 450-9D, 10D, and 11A1 react at different sites on the beta 4 molecule and MAbs 450-30A1 and 33D react at the same site on the alpha 6 subunit. Double determinant (two-site) radioimmunoassays using combinations of these MAbs have been developed. Two assays for beta 4 distinguish between the whole beta 4 molecule and the beta 4 molecule truncated from the C-terminus (form c) while another assay measures the presence of alpha 6 subunits. Data from the two-site assays support the following conclusions: (1) Colon tumors and normal colon mucosa express large amounts of alpha 6 beta 4 although only form c of the beta 4 was detected; (2) There is no evidence for alpha 6 beta 1 expression in colon; however, some of this complex may be present in certain lung tumors. The extracellular domains of alpha 6 and beta 4 can associate with each other even if the cytoplasmic domain of the beta 4 subunit is not present. MAbs to specific domains of the beta 4 molecule may be useful in analyses of forms a and c in normal and malignant tissue. The fact that only the largest beta 4 molecule "a" retains the phosphorylation site may have functional significance.

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