Radioligand racemic

Nagase and his collaborators have developed a series of indolo[2,3-g]octa-hydroisoquinoline derivatives mimicking the structure of NTI [196]. The compounds show in the MVD and radioligand binding studies antagonism and selectivity at 5 receptors. Racemic compound (128), for instance, exhibited in binding a <5 Ki value of 3.5 nM and a pild and k/d selectivity ratio of 205 and 272 respectively. Compound (128) did not affect the inhibition of contraction in the GPI and showed a Ke value in the MVD against DPDPE of 4.8 nM. 

Conjugation of a racemic drug to a protein increases the possibility for heterogeneous antisera because at least two haptens (R and S) are introduced. The immune system could respond by producing antibodies to only one enantiomer, or varying amounts of antibodies to each enantiomer. Each antibody may also have affinity for the opposite enantiomer. The use of racemic radioligands further complicates the situation. These problems have been discussed by Maeda and Tsuji (23), Cook et al. (24), and Rominger and Albert (25). 

Figure 3 Competition with binding of racemic tribum-labeled WR 171,669 expressed as counts per minute of radioligand bound as function of log of competitor weight (A) or reciprocal of counts per minute of radioligand bound as a funcHon of competitor weight (C). x = binding in absence of compebtor + = nonspecific binding B= d-, A = /, = d,/-WR 171,669. (From Ref. 23, used with permission.)...

The work with atropine presents a confusing picture. Use of racemic radioligand in these studies may contribute to the observed cross-reactivity picture. The rather facile enolization and hence racemization of atropine-type structures is another problem. FinaUy, the structure of the product of atropine and diazotized p-aminobenzoic acid has never been established conclusively. For these reasons, the radioreceptor assay for this compound (see below) is generally preferable. 

A racemic nicotine analog, fra s-3-hydroxymethyl-nicotine, was converted to the hemisuccinate, which was conjugated to protein to form an immunogen (Fig. 4 [8c]). The resulting antiserum was used with tiitiated 1-nicotine as radioligand. With this radioligand the assay was highly selective for 1-nicotine, with less than 0,01% cross-reaction with d-nicotine. Similar enantioselectivity is claimed for 1-cotinine (42). 

Aaltonen et al. (61) compared RRA and RIA for atropine. These workers obtained preparations of receptor from rat brain and lyophilized them to a stable, dry form, They used the tritium-labeled quinuclidinyl benzilate at 35 Ci/mmol. The affinity constant was 0.48 nM, and by analysis of 25- xL serum samples they could obtain a sensitivity down to 1.25 ng/mL in serum. Nonspecific binding was again quite reasonable (4%) and a filtration-type separation was used. The d isomer of an atropine did not bind, and therefore, the cross-reaction of the d,l compound was 50% that of the ( isomer. For comparison they used RIA developed by the method of Virtanen et al, (37). The immunogen was an /-hyoscyamine-bovine serum albumin conjugate, but the antiserum was sensitive to both d,l and I isomers. Racemic tritium-labeled atropine was used as the radioligand. 

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