Synthesis and biological properties of alpha-mono- and alpha-difluoromethyl derivatives of tryptophan and 5-hydroxytryptophan

The syntheses of alpha-mono- and alpha-difluoromethyl derivatives of tryptophan and 5-hydroxytryptophan are described. In an attempt to selectively regulate serotonin synthesis, alpha-(mono- and difluoromethyl)tryptophan were tested in vivo as precursors (or prodrugs) of their 5-hydroxy analogues. Although alpha-(mono- and difluoromethyl)-5-hydroxytryptophans are potent irreversible inhibitors of aromatic amino acid decarboxylase (equipotent to alpha-difluoromethyl-Dopa), only alpha-(monofluoromethyl)tryptophan affects the level of serotonin in vivo (small decrease), alpha-(difluoromethyl)tryptophan being a very poor substrate of the activating (or helper) enzyme, tryptophan hydroxylase.

Oxidation of tryptophan to oxindolylalanine by dimethyl sulfoxide-hydrochloric acid. Selective modification of tryptophan containing peptides

Tryptophan is readily oxidized to oxindolylalanine (2-hydroxytryptophan) in good yield on treatment in acetic acid solution with a mixture of dimethyl sulfoxide (DMSO) and concentrated aqueous HCl at room temperature. Other sulfoxides can be used in combination with HCl; for example, methionine sulfoxide reacts with an equimolar amount of tryptophan to give high yields of methionine and oxindolylalanine. Methionine and cysteine are quantitatively oxidized by DMSO/HCl to methionine sulfoxide and cystine, respectively. The tryptophan containing peptides LRF (luteinizing hormone-releasing factor), somatostatin, valine-gramicidin A and ACTH 1-24 were each treated with the DMSO/HCl reagent in acetic acid solution and the corresponding oxindolylalanine-derivatives isolated in over 90% yield after chromatography. The identity and purity of the derivatives were established on the basis of ultraviolet spectral characteristics and quantitative amino acid analysis of the oxindolylalanine content of acid hydrolyzates of the oxidized peptides with 3N-p-toluenesulfonic acid at 110 degrees for 24 h. The results indicate that modification of tryptophan peptides with DMSO/HCl provides a useful procedure, which seems superior to previously used reagents. In addition, the method could be well applied to other indoles of biological and pharmacological interest.

Enantiospecific syntheses of alpha-(fluoromethyl)tryptophan analogues: interactions with tryptophan hydroxylase and aromatic L-amino acid decarboxylase

alpha-Fluoromethyl amino acids are enzyme-activated irreversible inhibitors of amino acid decarboxylases. Aromatic L-amino acid decarboxylase (AADC) is the enzyme responsible for the final step in the biosynthesis of both dopamine and serotonin via decarboxylation of L-dopa and 5-hydroxy-L-tryptophan, respectively. Our goal is to utilize antagonists of the serotonin-producing enzymes (tryptophan hydroxylase and AADC) as the basis for a chemotherapeutic approach to the treatment of carcinoid tumors, a rare tumor type characterized by the overproduction of serotonin. We report here an enantiospecific synthesis of alpha(S)-(fluoromethyl)tryptophan [(S)-11a] and alpha(S)-(fluoromethyl)-5-hydroxytryptophan [(S)-11b], as well as the (R)-enantiomers, based upon recent methodology involving the face-selective alkylation of cyclic tryptophan tautomers. Our synthetic route provided both enantiomers of 11a and 11b with greater than 97% enantiomeric purity based upon evaluation of the NMR spectra of their Mosher's acid derivatives. (S)-11a was evaluated as a substrate for P815 tryptophan hydroxylase and determined to have an apparent Km of 4.31 +/- 1.07 mM, essentially half the value previously reported for the racemic mixture of 11a with rat brain stem tryptophan hydroxylase. (R)-11a was not a substrate for P815 tryptophan hydroxylase. (S)-11b was evaluated as an enzyme-activated irreversible inhibitor of murine liver AADC and determined to have a KI of 24.3 +/- 3.01 microM and a k2 of 2.26 +/- 0.44 min-1. (R)-11b was not an inhibitor of murine liver AADC.

Development of an automated synthesis apparatus for L-[3-11C] labeled aromatic amino acids

We have developed an automated synthesis apparatus for L-[3-11C]-labeled aromatic amino acids by improved multi-enzymatic reactions. Use of a specially designed reaction vessel and of CH2Cl2 as a solvent has greatly simplified the overall operation, proving to be suitable for automated synthesis, and also has given better yields and higher specific activities than formerly reported values.

Tissue Immunoassay for19F-Tagged 5-Hydroxytryptophan

A new tool for magnetic resonance, L-6-heptafluorobutyryl-5-hydroxytryptophan, was synthesized and investigated using an antibody to perfluoroalkyl moieties developed previously. To be useful as an imaging agent, the compound must cross the blood brain barrier and then be concentrated in vesicles in serotonergic neurons in order to accumulate in sufficient quantity for in vivo detection to be possible. The novel imaging compound was administered in ova to domestic chicks (Gallus domestics) to investigate the bioavailability and uptake dynamics of the compound in this model organism. Typical immunoassay methods were ineffective, so a new technique was developed which binds amines and amino acids to the walls of acid-functionalized cuvettes. The first study established the presence of higher quantities of the tags in neural and liver tissue than in heart tissue. A second study investigated regional differences, with the midbrain containing more tagged compounds than the frontal lobe sample, and the frontal lobe sample containing more than the occipital or cerebellum samples. These studies demonstrate that the compound follows the pathway of endogenous serotonin. A third study investigated uptake dynamics of the novel compound. Maximum concentration of the tagged molecule in the brain was achieved three days after injecting Incubation Day 14 eggs, suggesting that it bioaccumulates in vivo. This new immunoassay technique used to detect the novel compound in tissue samples demonstrated good repeatability.

Analgesic properties of a systemically-administered synthetic dipeptide of 5-hydroxytryptophan

Synthetic peptides of 5-hydroxytryptophan (5-HTP), including N-acetyl-5-HTP-5-HTP amide (5-HTP-ACETYL-DP), specifically inhibit the binding of serotonin to serotonin binding protein. 5-HTP-ACETYL-DP also produces a long-lasting, opiate-sensitive analgesia following central, but not systemic administration. The present study evaluated an apolar derivative of 5-HTP dipeptide, N-hexanoyl-5-HTP-5-HTP amide (5-HTP-HEX-DP), for its analgesic properties in rats following systemic administration. 5-HTP-HEX-DP (5-50 mg/kg) significantly increased jump thresholds in a dose-dependent manner with peak analgesia occurring at 2.5 hr after injection, and lasting up to 5 hr. In the tail-flick assay, 5-HTP-HEX-DP (20 mg/kg) produced a significant antinociceptive effect at 1 hr post-injection using both high and low intensity levels of radiant heat. While 5-HTP-HEX-DP and morphine each elicited analgesia following acute administration, chronic (14 days) incremental dosing with 5-HTP-HEX-DP or morphine resulted in persistent analgesia in 5-HTP-HEX-DP-treated animals, and a loss of analgesia in morphine-treated rats. Thus, significant tolerance to morphine, but not 5-HTP-HEX-DP analgesia developed using this protocol. Hence, 5-HTP-HEX-DP is a systemically-active analgesic which fails to develop tolerance when administered daily over 14 days.