L-α-Aminopimelic acid

Analogues of the putative excitatory transmitters aspartic acid and glutamic acid were tested for antagonism against stimulus-evoked activation of Schaffer collateral-CA 1 pyramidal cell synapses in slices of rat hippocampus. Responses to the analogues, applied via the superfusing medium, were extracellularly recorded. The compounds examined included D- and L-alpha-aminodicarboxylic acids, diaminodicarboxylic acids, phosphonate analogues of acidic amino acids, D- and L-gamma-glutamyl glycine, and the cis- and trans-isomers of piperidine 2,3-, and 2,4-dicarboxylic acid. Many of these compounds are known to be potent and selective antagonists for excitatory amino acids and a few excitatory pathways. In this hippocampal pathway most of these analogues showed relatively low and similar potency. The most potent antagonist uncontaminated with agonist activity was D-alpha-aminosuberate, with an apparent antagonist dissociation constant (Kd) of 3 mM. Only 5 of the analogues, 3 of the piperidine dicarboxylates, kainic acid, and L-alpha-aminopimelic acid, reduced the amplitude of the extracellularly recorded field potentials more than 30% at 0.5 mM. However, all of the others reduced the potential by more than 30% at 5 mM. Most analogues also evoked extracellular responses which can be attributed to depolarization of the pyramidal neurons. Agonist activity was particularly strong among the most potent analogues. These results contrast with the responses documented by others for the N-methyl-D-aspartate receptor of the dorsal-ventral root excitatory pathway of the spinal cord in which the higher homologues tested here were the most potent antagonists, and the D-isomers were more potent than the L-isomers. It also contrasts with the response of the perforant path synapses to granule cells of the dentate gyrus in which the portion derived from the lateral entorhinal cortex is sensitive to L-2-amino-4-phosphonobutyric acid. Thus the Schaffer-CA 1 pyramidal cell synaptic field utilizes a novel excitatory transmitter receptor which interacts detectably but only weakly with a variety of acidic amino acids with potent specific inhibitory action for receptors elsewhere in the central nervous system.

Chain-extended analogues of methotrexate were synthesized by condensation of 4-amino-4-deoxy-N10-methylpteroic acid with esters of L-alpha-aminoadipic, L-alpha-aminopimelic, and L-alpha-aminosuberic acids, followed by ester hydrolysis with acid or base. Coupling was accomplished in up to 85% yield by the use of the peptide bond forming reagent diethyl phosphorocyanidate at room temperature. The products were found to bind bacterial (Lactobacillus casei) and mammalian (L1210 mouse leukemia) dihydrofolate reductase with an affinity comparable to methotrexate and were also equitoxic to L1210 cells in culture. Cytotoxicity increased up to 3-fold as the number of CH2 groups in the amino acid side chain was extended from two to five. The alpha-aminoadipate and alpha-aminopimelate analogues were poor substrates for carboxypeptidase G1, confirming that this enzyme has a strict requirement for a C-terminal L-glutamic acid residue. The in vivo antitumor activity of the chain-extended analogues against L1210 leukemia in mice was comparable to that of the parent drug on the qd X 9 schedule, but higher doses were required to achieve the same increase in survival. The results were consistent with findings, reported separately, that these compounds are poor substrates for folate polyglutamate synthetase and therefore would not be expected to form gamma-polyglutamates once they enter a cell. This distinctive property has potential therapeutic implications for the treatment of certain MTX-resistant tumors whose resistance may be associated with a lower than normal capacity to form gamma-polyglutamates in comparison with proliferative tissues such as intestinal mucosa or marrow.