Conjugation with L-Glutamate for in vivo brain drug delivery

In vitro studies have shown that conjugation of a model compound [p-di(hydroxyethyl)-amino-D-phenylalanine (D-MOD)] with L-Glu can improve D-MOD permeation through the bovine brain microvessel endothelial cell monolayers (Sakaeda et al., 2000). The transport of this D-MOD-L-Glu conjugate is facilitated by the L-Glu transport system. In this paper, we evaluate the in vivo brain delivery of model compounds (i.e. D-MOD, p-nitro-D-phenylalanine (p-nitro-D-Phe), 5,7-dichlorokynurenic acid (DCKA) and D-kyotorphin) and their L-Glu conjugates. DCKA was also conjugated with L-Asp and L-Gln amino acids. The analgesic activities of D-kyotorphin and its L-Glu conjugate were also evaluated. The results showed that the brain-to-plasma concentration ratio of D-MOD-L-Glu was higher than the D-MOD alone; however, the plasma concentration of both compounds were the same. The plasma concentration of p-nitro-D-Phe-L-Glu conjugate was higher than the parent p-nitro-D-Phe; however, the brain-to-plasma concentration ratio of p-nitro-D-Phe was higher than its conjugate. On the other hand, both DCKA and DCKA conjugates have a low brain-to-plasma concentration ratio due to their inability to cross the blood-brain barrier (BBB). The L-Asp and L-Glu conjugates of DCKA have elevated plasma concentrations relative to DCKA; however, the DCKA-L-Gln conjugate has the same plasma concentration as DCKA. For D-kyotorphin, both the parent and the L-Glu conjugate showed similar analgesic activity. In conclusion, conjugation of a non-permeable drug with L-Glu may improve the drug's brain delivery; however, this improvement may depend on the physicochemical and receptor binding properties of the conjugate.

Modulation of melphalan resistance in glioma cells with a peripheral benzodiazepine receptor ligand-melphalan conjugate

Peripheral benzodiazepine receptors (PBRs) are located on the outer membrane of mitochondria, and their density is increased in brain tumors. Thus, they may serve as a unique intracellular and selective target for antineoplastic agents. A PBR ligand-melphalan conjugate (PBR-MEL) was synthesized and evaluated for cytotoxicity and affinity for PBRs. PBR-MEL (9) (i.e., 670 amu) was synthesized by coupling of two key intermediates: 4-[bis(2-chloroethyl)-amino]-L-phenylalanine ethyl ester trifluoroacetate (6) and 1-(3'-carboxylpropyl)-7-chloro-1,3- dihydro-5-phenyl-2H-1,4-benzodiazepin-2-one (8). On the basis of receptor-binding displacement assays in rat brain and glioma cells, 9 had appreciable binding affinity and displaced a prototypical PBR ligand, Ro 5-4864, with IC50 values between 289 and 390 nM. 9 displayed differential cytotoxicity to a variety of rat and human brain tumor cell lines. In some of the cell lines tested including rat and human melphalan-resistant cell lines, 9 demonstrated appreciable cytotoxicity with IC50 values in the micromolar range, lower than that of melphalan alone. The enhanced activity of 9 may reflect increased membrane permeability, increased intracellular retention, or modulation of melphalan's mechanisms of resistance. The combined data support additional studies to determine how 9 may modulate melphalan resistance, its mechanisms of action, and if target selectivity can be achieved in in vivo glioma models.

Derivatives of melphalan designed to enhance drug accumulation in cancer cells

The objective of this study was to develop chemical strategies to improve the uptake and accumulation of melphalan (L-Mel and D-Mel), a cytotoxic agent, into cancer cells. Dipeptides synthesized from L- (or D-) Mel and L-glutamic acid (L-Glu) or L-valine (L-Val) and their methyl or ethyl esters (all compounds were trifluoroacetic acid salts) were evaluated for cytotoxicity and cellular uptake using Caco-2 cells, a human colon carcinoma cell line, and RT-2 cells, a rat brain glioma cell line. Treatment of Caco-2 cells with L-Mel or D-Mel (0.5 mg/ml equivalent of melphalan) for 48 h resulted in approximately 50% cell survival. Treatment of the Caco-2 cells with dipeptide derivatives of L-Mel (or D-Mel) (11c-d, 12c-d and 13) caused similar cytotoxicity effects (approximately 50-70% of cell survival). When the cytotoxicities of the esters of L-Mel, D-Mel and their dipeptide derivatives (11a-b, 12a-b and 14) in Caco-2 cells were determined, less than 10% cell survival was observed. Similar results were observed in RT-2 cells. When the cellular uptake properties of these compounds were determined in Caco-2 cell monolayers, L-Glu-L-Mel (12c), L-Glu-D-Mel (12d), and L-Mel-L-Glu (11c) generated slightly lower intracellular levels of L-Mel or D-Mel than when the cell monolayer was treated with the amino acids (L-Mel or D-Mel). In Caco-2 cells treated with 11c, 12c or 12d, low levels of the dipeptides were also detected. Caco-2 cell monolayers treated with D-Mel-L-Glu (11d) or D-Mel-L-Val (13) showed very low levels of the amino acids (L-Mel or D-Mel), but generally higher levels of the dipeptides. In contrast to the amino acids (L-Mel, D-Mel) or the dipeptide derivatives (11c-d, 12c-d and 13), the ester derivatives of the amino acids [L-Mel(OEt), D-Mel(OEt)] or the dipeptides (11a-b, 12a-b and 14) produced 5-20 times higher intracellular concentrations of potentially cytotoxic metabolites (e.g., L-Mel, D-Mel, Mel-containing dipeptides or Mel-containing dipeptide monoesters). L-Mel(OEt), D-Mel(OEt), L-Glu(OEt)-L-Mel(OEt) (12a), L-Glu(OEt)-D-Mel(OEt) (12b), and L-Mel-L-Glu(OEt)2 (11a) accumulated mainly as either L-Mel or D-Mel, and the percentages of L-Mel or D-Mel were 99%, 99%, 90%, 75% and 98% of the total intracellular concentration of potentially cytotoxic agents, respectively. D-Mel-L-Glu(OEt)2 (11b) accumulated as its monoester (> 95%) and D-Mel-L-Val(OMe) (14) accumulated as its dipeptide metabolite (> 98%). Inclusion of Gly-Pro, carnosine, L-Phe or L-Glu did not inhibit uptake of the dipeptide derivatives of L-Mel (or D-Mel) or their esters. These results suggest that the cellular uptake of the dipeptide derivatives of melphalan and their esters is probably via passive diffusion rather than being facilitated by an amino acid transporter or a di/tripeptide transporter. The higher intracellular levels of cytotoxic agents generated from the ester derivatives of the amino acids and the dipeptides are probably due to their higher lipophilicity and the overall neutral charge of the esters and subsequent intracellular formation of the more polar amino acids (L- or D-Mel) and/or Mel-containing dipeptides. Finally, these studies suggest that dipeptides of D-Mel [11b, 11d, 13] have inherent cytotoxicity properties.

2-amino-3-phosphonopropionic acid (AP3) and its N-methyl analog improve learning and memory in rats

The influence of 2-amino-3-phosphonopropionic acid (AP3), an antagonist of metabotropic glutamate receptors and its N-methyl analog (N-methyl AP3) on acquisition, consolidation and recall of information in a passive avoidance situation was tested. Both compounds improved three examined parameters. The significant effect of AP3 appeared at the icv dose of 12.5 micrograms per rat when its influence on acquisition and consolidation was tested, and at the icv dose of 100 micrograms when its influence on retrieval process was evaluated. The significant improvement of acquisition of information was observed at the icv dose of 50 micrograms per rat of N-methyl AP3, but significant facilitation of consolidation and retrieval processes appeared at the higher dose of 100 micrograms per rat. Significant improvement of performance in a passive avoidance situation persisted in all experiments to the highest used dose of both compounds, 250 micrograms.

Central activity of 2-amino-3-phosphonopropionic acid (AP3) and its N-methyl analog

The influence of 2-amino-3-phosphonopropionic acid (AP3) an antagonist of metabotropic glutamate receptors and its N-methyl analog (N-methyl AP3) on locomotor activity in the "open field" test, apomorphine stereotypy and haloperidol catalepsy were tested. Both compounds had not any influence on spontaneous crossings of squares, number of rearings and bar approaches in the "open field" test. AP3 and its N-methyl analog significantly enhanced apomorphine stereotypy at the intracerebroventricular (icv) doses of 100 and 200 micrograms per rat respectively. Although, both compounds significantly diminished haloperidol catalepsy at the icv doses of 50, 100 and 200 micrograms per rat. All observed effects of AP3 and N-methyl AP3 were dose independent. Methyl substitution at the N-position in AP3 did not change pharmacological properties of this compound i.e. its influence on behavior mainly dependent on dopaminergic systems.

Synthesis of phosphonic analogs of enkephalins. Pentapeptides with COOH replaced by PO3H2 group

Eight analogs of enkephalins containing the phosphonic analogs of Gly, Leu, Met, Phe and Pro have been synthesized by standard procedures in solution. The key intermediates in the synthesis were pure diastereomers of dialkyl N-L-phenylalanyl/aminoalkanephosphonates obtained from racemic dialkyl aminoalkanephosphonates by coupling with phenylalanine and resolution of the resulting mixture of L, D and L, L dipeptides by means of column chromatography.