Biotin reagents for antibody pretargeting. 4. Selection of biotin conjugates for in vivo application based on their dissociation rate from avidin and streptavidin

An investigation was conducted to determine the affect of structural variation of biotin conjugates on their dissociation rates from Av and SAv. This information was sought to help identify optimal biotin derivatives for in vivo applications. Fifteen biotin derivatives were conjugated with a cyanocobalamin (CN-Cbl) derivative for evaluation of their "relative" dissociation rates by size exclusion HPLC analysis. Two biotin-CN-Cbl conjugates, one containing unaltered biotin and the other containing iminobiotin, were prepared as reference compounds for comparison purposes. The first structural variations studied involved modification of the biotinamide bond with a N-methyl moiety (i.e., sarcosine conjugate), lengthening the valeric acid side chain by a methylene unit (i.e., homobiotin), and replacing the biotinamide bond with thiourea bonds in two conjugates. The rate of dissociation of the biotin-CN-Cbl derivative from Av and SAv was significantly increased for biotin derivatives containing those structural features. Nine additional biotin conjugates were obtained by coupling amino acids or functional group protected amino acids to the biotin moiety. In the conjugates, the biotin moiety and biotinamide bond were not altered, but substituents of various sizes were introduced alpha to the biotinamide bond. The results obtained from HPLC analyses indicated that the rate of dissociation from Av or SAv was not affected by small substituents alpha to the biotinamide (e.g., methyl, hydroxymethyl, and carboxylate groups), but was significantly increased when larger functional groups were present. On the basis of the results obtained, it appears that biotin conjugates which retain an unmodified biotin moiety and have a linker molecule conjugated to it that has a small functional group (e.g., hydroxymethylene or carboxylate) alpha to the biotinamide bond are excellent candidates for in vivo applications. These structural features are obtained in the biotin amino acid conjugates: biotin-serine, biotin-aspartate, biotin-lysine, and biotin-cysteine. Importantly, these biotin derivatives can be readily conjugated with other molecules for specific in vivo applications. In our studies, these derivatives will be used in the design of new biotin conjugates to carry radionuclides for cancer therapy using the pretargeting approach.

Biotin reagents for antibody pretargeting. Synthesis, radioiodination, and in vitro evaluation of water soluble, biotinidase resistant biotin derivatives

As part of our development of antibody pretargeting for cancer therapy, an investigation has been conducted to examine the stability of water solubilized, radioiodinated biotin derivatives toward biotinidase degradation in mouse and human serum. Eight new biotin derivatives were synthesized to conduct the study. The biotin derivatives synthesized contained (1) the biotin moiety, (2) a water solubilizing linker moiety, (3) p-iodobenzoate or p-tri-n-butylstannylbenzoate moieties, and (4) in some of the compounds, N-methyl or alpha-methyl containing moieties were added to block biotinidase activity. The linker moiety, 4,7,10-trioxa-1,13-tridecanediamine, 5, was included in the biotin derivatives to improve their water solubility, and it also functioned as a 17 A spacer between the biotin and the benzoyl moieties. Four of the new biotin derivatives (12, 14, 22, and 23) contained a p-tri-n-butylstannylbenzoyl moiety as precursors which could be radioiodinated in the last synthetic step. The other four biotin derivatives (13, 15, 24, and 25) contained p-iodobenzoyl moieties and were used as HPLC reference standards. Initial studies involved radioiodination of 12 to yield [125I]13. Radioiodinated 13, which did not contain a moiety for blocking biotinidase activity, was found to be rapidly degraded in both mouse and human serum at 37 degrees C. Derivatives which were designed to be stable to biotinidase incorporated N-methyl and alpha-methyl moieties adjacent to the biotin carboxylate group. In one set of biotin derivatives (14 and 15), the N-methyl moiety was obtained by incorporating N,N-dimethyl-4,7,10-trioxa-1,13-tridecanediamine, 9, as a linker in the place of 5. In the second set of biotin derivatives (22 and 24), the N-methyl moiety was introduced by incorporating a sarcosine (N-methylglycine) moiety between biotin and 5. The radioiodinated N-methyl containing biotin derivatives [125I]15 and [125I]24 were found to be very stable to biotinidase degradation. An alpha-methyl group was obtained in a pair of biotin derivatives (23 and 25) by incorporating a 3-aminobutyric acid moiety between biotin and 5. The radioiodinated alpha-methyl containing derivative, [125I]25, was found to have an intermediate stability with regards to biotinidase degradation.

Synthesis and preliminary biological evaluation of (3-iodobenzoyl)norbiotinamide and ((5-iodo-3-pyridinyl)carbonyl)norbiotinamide: two radioiodinated biotin conjugates with improved stability

A new class of radioiodinated biotin conjugated is described in which the amido bond between biotin and the labeled prosthetic group is reversed. One conjugate, (3-[125I]iodobenzoyl)norbiotinamide (4c, [125I]IBB) was labeled with Na125I in one step from (3-(tributylstannyl)benzoyl)norbiotinamide (4b, TBB) via a demetalation reaction. However, the analogous reaction with ((5-(tributylstannyl)-3-pyridinyl)carbonyl)norbiotinamide (6b, TPB) failed to yield ((5-[131I]iodo-3-pyridinyl)carbonyl)norbiotinamide (6c, [131I]IPB, necessitating a two-step approach for synthesizing [131I]IPB. The binding of [125I]IBB and [131I]IPB to streptavidin in vitro was identical to that of biotinyl-3-[125I]iodoanilide, a conjugate with an amido bond with normal configuration. Both [125I]IBB and [131I]IPB were stable in serum while the first-generation compound was rapidly degraded. The biodistribution patterns of [125I]IBB and [131I]IPB in mice are consistent with limited degradation of these conjugates by biotinidase and deiodinases.

Preparation and biological evaluation of an astatine-211 labeled biotin conjugate: biotinyl-3-[211 At]astatoanilide

Biotinyl-3-[211 At]astatoanilide ([211 At]AtBA) was prepared in more than 80% yield by destannylation. In vitro, [211 At]AtBA exhibited a high affinity for streptavidin, and was stable after incubation in human serum, cerebrospinal fluid and distilled water, whereas it was rapidly degraded in mouse serum. HPLC analysis showed that the main degradation pathway in mouse serum was the cleavage of [211 At]astatoaniline. In mice, [211 At]AtBA and its 125I-labeled analogue cleared rapidly from most tissues; however, there was some evidence for dehalogenation of both tracers.