Intracavitary therapy of murine ovarian cancer with cis-diamminedichloroplatinum(II) and 10-ethyl-10-deazaaminopterin incorporating systemic leucovorin protection.

Administration i.p. of 10-ethyl-10-deazaaminopterin (10EDAM) with cis-diamminedichloroplatinum(II) (cis-Pt) had significant antitumor activity against the murine ovarian tumor. This tumor is a teratoma originating in the ovary with pathogenesis and metastatic properties similar to those of human ovarian cancer. Drug was given on a schedule of once every 3 days for 3 doses 1 or 2 days after i.p. implant of 10(7) tumor cells. Despite the 2-fold attenuation of dosage required, antitumor activity of the combination (increased life span, 161%) was approximately twice that obtained with maximum tolerated doses of either agent alone and tumor-free, long-term survivors were obtained. Incorporation of s.c. calcium leucovorin administration 16 h after each dose of 10EDAM and cis-Pt allowed a 4-fold increase in dosage of 10-EDAM without an increase in toxicity, increased median survival by an additional 120%, and quadrupled the number of tumor-free, long-term survivors to 40% of treated animals. By comparison, methotrexate was only modestly active against this tumor model either as a single agent, with cis-Pt, or with delayed s.c. calcium leucovorin administration. These results appear to suggest that 10EDAM with cis-Pt may have considerable potential for intracavitary therapy of human cancer, including ovarian carcinoma, particularly when incorporating delayed systemic calcium leucovorin administration.

Similar specificity of membrane transport for folate analogues and their metabolites by murine and human tumor cells: a clinically directed laboratory study.

Mediated transport of folate compounds exhibited similar kinetic characteristics and structural specificity in a series of cultured murine and human tumor cells examined in a parallel fashion. In each case, influx was characterized by a single saturable component with an approach to steady-state conforming to a single exponential, while efflux was first order (poorly saturable). Both mediated fluxes exhibited high temperature dependence (Q10 27-37 degrees = 6 to 8). During competition studies with various analogues, it was found that positions 4, 5, 7, and 10 and the gamma-carboxyl position of the folate molecule were specified for influx in tumor cells from each species. Also, short-chain alkyl substitution at position 10 was specified in the case of N10, but not in the case of C10. None of the modifications at position 10 affected mediated efflux in either cell type. The linkage of additional glutamyl residues at the gamma-carboxyl-position resulted in reduced saturability (increased value for Ki) of influx in both murine and human tumor cells in a manner proportional to the number of glutamyl residues. Mediated influx in human ovarian carcinoma cells obtained from malignant effusions in several patients and in an established cell line derived from one of these patients showed similar kinetics for folate analogue transport and specificity for modification at position 10 of the 4-amino-folate molecule. Mediated entry of 10-deazaaminopterin and its 10-ethyl derivative compared to entry of methotrexate was 4- to 11-fold greater in murine tumor cells and 4- to 9-fold greater in human tumor cells in culture or when clinically derived. Mediated efflux was not specified for position 10 on the 4-amino folate structure in any tumor cell type. These findings appear to provide some basis for concluding that the results of studies of this type in model murine systems or with tumor cell lines established in culture have relevance to clinical cancer.

Toxicity, elimination, and metabolism of 10-ethyl-10-deazaaminopterin in rats and dogs.

10-Ethyl-10-deazaaminopterin (10-EdAM) is an antifolate compound with greater therapeutic activity than methotrexate against transplanted tumors in mice. When given weekly for 3 weeks, the 10% lethal dose in rats was 125 mg/kg (i.p.) and in dogs it was 2.5 mg/kg (i.v.). The major histopathological findings in intoxicated animals were damage to the mucosa of the gastrointestinal tract in rats and dogs and hypocellularity of the marrow in rats. The elimination of 50 mg/kg of 10-EdAM from the plasma of rats was triexponential with a terminal phase t1/2 of 18.5 h but a mean residence time of 0.7 h. The primary route of elimination in rats was biliary secretion of parent compound and eventual excretion of the parent compound and the deglutamate metabolite in the feces; the 7-hydroxy metabolite was also present in plasma, bile, and feces. Biliary elimination was independent of dose over a 5-fold range. The elimination of 10-EdAM from the plasma of dogs was also triexponential with a mean terminal phase t1/2 of 9.1 h and a mean residence time of 2.5 h; nonrenal clearance was the primary route of elimination. The pharmacokinetic parameters were independent of dose over the range of 0.25 to 5.0 mg/kg. High tissue concentrations of 10-EdAM were observed initially in liver, kidney, and small intestine of rats, while concentrations in bone marrow were low. Some polyglutamate formation was observed in these tissues as early as 0.5 h after drug administration but declined over 72 h.

An improved method for O-demethylation of codeine.

The O-demethylation of codeine was effected by sodium propylmercaptide in dimethylformamide at 125 degrees C to afford morphine in 80% yield. Similar treatment of thebaine was unrewarding.

Stabilized analogs of thymopentin. 3. Evaluation of ketomethylene pseudopeptides for antiarthritic properties.

This study analyzed the role of ketomethylene pseudopeptides of thymopentin as potential agents for the treatment of arthritis. The analogs were tested in vivo using assessment of inflammation and antibody production in the mouse type II collagen arthritis model and the rat adjuvant arthritis model. The compounds were also tested for immune-potentiating activity in vitro using induction of the lymphocyte marker, Thy-1.2, in mouse spleen cells and stimulation of T-cell proliferation. The results show that certain of the compounds exhibit disease-remitting properties for arthritis as evidenced by reduction of paw swelling in the mouse and rat models and decreased incidence of disease in the mouse model. The active compounds were dose specific and represented a range in efficacy. In spite of effects on arthritis, type II collagen antibody levels were not altered in the mouse model. Selected compounds also exhibited immune potentiating properties as evidenced by induction of Thy-1.2 expression and stimulation of T-cell proliferation. The absence of effect of the compounds on type II collagen antibody production suggests that the antiarthritic activity of the effective compounds results from alteration of cell-mediated immunity.

Potential histidine decarboxylase inhibitors. 1. alpha- and beta-substituted histidine analogues.

Histidine analogues with alkyl substitution at Calpha and Cbeta were prepared as potential inhibitors of specific histidine decarboxylase. Activity was assessed in vitro using extracts of rat pyloric stomach and a radioisotopic assay of 14CO2 evolved from carboxyl-14C-labeled histidine. alpha-Substituted analogues (C2-C4) including 2-hydroxyethyl were less potent than alpha-methylhistidine; the alpha-n-butyl analogue was completely inactive at 10(-3) M. Similarly, beta,beta-dimethylhistidine and homohistidine failed to exhibit activity at 10(-3) M.

Synthesis and antifolate properties of 5,10-ethano-5,10-dideazaaminopterin.

2-Carbomethoxy-4-(p-carbomethoxyphenyl)cyclohexanone was prepared in a four-step process and thermally condensed with 2,4,6-triaminopyrimidine to afford methyl 2,4-diamino-4-deoxy-7-hydroxy-5,10-ethano-5,10-dideazapteroate+ ++. Reduction of the 7-oxo function with borane gave the 7,8-dihydro pterin which was subsequently oxidized to the fully aromatic pteroate ester with dicyanodichlorobenzoquinone. Saponification of the benzoate ester, coupling with diethyl glutamate and final ester hydrolysis afforded the title compound. This novel deazaaminopterin analogue was approximately as potent as methotrexate in vitro in terms of DHFR and L1210 cell growth inhibition. There are indications of diastereomeric differences in the enzyme inhibition measurements. A significant transport advantage over MTX for influx into L1210 cells was observed. The compound was active against the E 0771 murine mammary solid tumor, but further investigation with individual diastereomers is required to define the ED50.

Effects of addition of a 2-methyl group to ethyl nipecotates (beta-meperidines) on receptor affinities and opiate agonist/antagonist activities.

A series of 2-methyl-3-carbethoxy-3-(m-hydroxyphenyl)piperidine opiates (13a-d) with N-substituent variations have been synthesized, and their receptor affinities and in vivo agonist and antagonist activities and energy-conformational profiles have been determined. These are racemates of the alpha-epimer at the C-2 position, with a methyl group cis to the 3-phenyl group. One of the main goals of this study was to compare the conformational and pharmacological behavior of these 2-methyl "beta-meperidine" analogues to their 2-desmethyl racemic counterparts (14a-c) previously reported in the literature. The 2-desmethyl and 2-methyl analogues were found to have very similar phenyl equatorial conformers as their lowest energy forms with the addition of a 2-methyl group diminishing conformational flexibility. The presence of the 2-methyl group appears to diminish affinity at the mu-receptor and also to somewhat diminish already weak antinociceptic agonist activity. Given the similarity in lowest energy conformation, this reduction is most likely caused by the unfavorable interaction of the methyl group itself with a local mu-receptor binding site. Superposition of the phenol OH and protonated amine nitrogen NH of either 2-methyl enantiomer of 13a in its lowest energy conformer with the same OH and NH groups of metazocine, used as a high affinity rigid analogue, leads to reasonable overlap. However, the N-substituents and the piperidine and phenyl rings do not overlap in this proposed pharmacophore, perhaps accounting for the rather poor affinities found for these 3-phenylpiperidines and the lack of N-substituent modulation of affinity and efficacy as in fused ring opioids.

Synthesis and antitumor activity of 10-alkyl-10-deazaminopterins. A convenient synthesis of 10-deazaminopterin.

Requirements for large-scale synthesis of the potent antitumor drug 10-deazaminopterin have led to development of a facile synthesis of this compound and its 10-alkyl analogues. The lithium diisopropyl amide generated dianions of appropriate p-alkylbenzoic acids were alkylated with 3-methoxyallyl chloride. The resulting 4-(p-carboxyphenyl)-1-methoxy-1-butenes were brominated at pH 7-8 to afford the 2-bromo-4-(p-carboxyphenyl)butyraldehydes. Condensation with 2,4,5,6-tetraminopyrimidine and subsequent in situ oxidation of the resulting dihydropteridines yielded crystalline 10-alkyl-10-deaza-4-amino-4-deoxypteroic acids. The pteroic acids were coupled with diethyl glutamate via the mixed anhydride method, followed by saponification at room temperature, to give the target 10-deazaminopterins. The 10-alkyl compounds were approximately equipotent to 10-deazaminopterin as growth inhibitors of folate-dependent bacteria. Their abilities to inhibit Lactobacillus casei and L1210 derived dihydrofolate reductases were also similar. Transport properties in vitro were suggestive of an improved therapeutic index for the 10-alkyl analogues. Against L1210 in mice, the percent increase in life span at the LD10 dosage was +151% (methotrexate), +178% (10-deazaminopterin), +235% (10-methyl analogue), and +211% (10-ethyl analogue). 10,10-Dimethyl-10-deazaminopterin was less effective at an equimolar dosage, but the ILS at the maximum dose tested (72 mg/kg) was +135%. It was far less toxic than the other analogues possibly because of enhanced clearance.

Synthesis and antifolate properties of 10-alkyl-5,10-dideaza analogues of methotrexate and tetrahydrofolic acid.

Synthesis of the 10-methyl and 10-ethyl analogues of 5,10-dideazatetrahydrofolic acid (DDTHF), a potent inhibitor of glycinamide ribotide (GAR) formyltransferase, is reported. Key intermediates in the process were 10-methyl- and 10-ethyl-4-amino-4-deoxy-5,10-dideazapteroic acid. Condensation of the piperidine enamines of branched 4-(p-carbomethoxyphenyl)butyraldehydes with (acetoxymethylene)malononitrile afforded 1,1-dicyano-4-piperidinobutadiene 5a,b. Subsequent reaction with alcoholic ammonium hydroxide yielded the appropriately substituted 2-amino-3-cyanopyridines 6a,b. Ring closure with guanidine gave 10-methyl- and 10-ethyl-4-amino-4-deoxy-5,10-dideazapteroic acids (7a,b). Coupling with diethyl glutamate followed by ester hydrolysis afforded 10-alkyl-5,10-dideazaminopterin analogues 9a,b. Hydrolysis of the 4-amino group of 7a,b yielded the 10-alkylpteroic acids, which were coupled with diethyl glutamate, hydrogenated over PtO2, and saponified to afford 10-alkyl-5,10-dideazatetrahydrofolic acids 13a,b. Aminopterin analogues 9a,b were effective inhibitors of DHFR derived from L1210, but were less potent than methotrexate for inhibition of growth of L1210 in culture. The 10-ethyl (13b) analogue of 5,10-DDTHF was about twice as potent an inhibitor of L1210 cell growth as 5,10-DDTHF, but was only 1/7 as potent for inhibition of GAR formyltransferase. 10-Methyl analogue 13a was similar in potency to 5,10-DDTHF. All of the compounds showed moderately improved transport into L1210 cells relative to methotrexate.

Synthesis and antifolate properties of 9-alkyl-10-deazaminopterins.

Reformatski condensation of benzyl 2-bromopropionate with 4-carbomethoxybenzaldehyde, followed by dehydration afforded benzyl 2-methyl-p-carbomethoxycinnamate (4a). Hydrogenation over a Pd catalyst gave the hydrocinnamic acid 5a. Conversion to the chloromethyl (6a) and azidomethyl ketone (7a) was followed by hydrogenation to the aminomethyl ketone (8a). Direct N-alkylation by 2,4-diamino-5-nitro-6-chloropyrimidine followed by reductive ring closure in Zn-HOAc and subsequent saponification of the benzoate ester yielded 4-amino-4-deoxy-9-methyl-10-deazapteroic acid (11a). Coupling with diethyl L-glutamate and saponification afforded 9-methyl-10-deazaminopterin (13a). The 9-ethyl analogue (13b) was similarly prepared from benzyl 2-bromobutyrate. The 9-methyl analogue (13a) was 21 times more potent than MTX as an inhibitor of cell growth in L1210 cells. The reason for this enhanced cytotoxicity in L1210 is unclear, since enzyme inhibition and transport parameters were similar to those of MTX. In human Manca leukemia cells growth inhibition was not dramatic and paralleled MTX.

Synthesis and antifolate properties of 5,10-methylenetetrahydro-8,10-dideazaminopterin.

The synthesis of the 5,10-methylene analogue of 5,6,7,8-tetrahydro-8,10-dideazaminopterin, a potential dual inhibitor of dihydrofolate reductase (DHFR) and thymidylate synthase (TS) enzymes, is described. The dimethyl ester of 10-carboxy-4-amino-4-deoxy-8,10-dideazapteroic acid was converted to the tetrahydro derivative by hydrogenation. Thermally induced cyclization of the 10-carbomethoxy and the 5-NH groups afforded the 5,10-carbonyl analogue. Reduction of the lactam with borane readily yielded the key 5,10-methylene-4-amino-4-deoxy-8,10-dideazatetrahydropteroic acid methyl ester. Saponification of the benzoate ester and coupling with L-glutamate concluded the synthesis. The title compound was a modest inhibitor of growth in folate-dependent bacteria. Streptococcus faecium and Lactobacillus casei, but inhibition of DHFR or TS derived from L. casei was poor. The compound was also a weak inhibitor of DHFR derived from L1210 murine leukemia and was a weak inhibitor of L1210 growth in culture.

Synthesis and antitumor activity of 10-propargyl-10-deazaaminopterin.

Successive alkylation of dimethyl homoterephthalate with propargyl bromide and 2,4-diamino-6-(bromomethyl)pteridine followed by ester saponification at room temperature afforded 2,4-diamino-4-deoxy-10-carboxy-10-propargyl-10-deazapteroic acid. The 10-COOH was readily decarboxylated by heating in DMSO at a temperature of only 120 degrees C to yield the diamino-10-propargyl-10-deazapteroic acid intermediate. Coupling with diethyl L-glutamate and ester hydrolysis gave the title compound. The 10-propargyl analogue was about 5 times more potent than MTX as an inhibitor of growth in L1210 cells, but was only one-third as potent as an inhibitor of DHFR from L1210. The analogue was transported inward very effectively in L1210 cells showing a 10-fold advantage over MTX. At a dose of 36 mg/kg the 10-propargyl compound caused shrinkage of the E0771 solid murine mammary tumor to only 1% of untreated controls.

Synthesis and biological evaluation of 8-deazahomofolic acid and its tetrahydro derivative.

The syntheses of 8-deazahomofolic acid and its tetrahydro derivative, potential inhibitors of thymidylate synthase (TS) and other folate related enzymes, are described. Wittig condensation of 2-acetamido-6-formyl-4-pyrimidinol with the triphenylphosphine ylide 3 derived from N-acetyl-4-(p-carbethoxyanilino)-1-chloro-2-butanone, hydrogenation of the enone intermediate 5, introduction of a 5-amino group via diazonium coupling, and reductive ring closure yielded ethyl N11-acetyl-8-deazahomopteroate (8). Alkaline hydrolysis gave 8-deazahomopteroic acid, which was blocked as the 11-trifluoroacetyl derivative, coupled with diethyl L-glutamate, and the blocking groups saponified to afford 8-deazahomofolic acid (12). Hydrogenation of the glutamate diester intermediate and subsequent saponification yielded the tetrahydro-8-deazahomofolate (14). Growth inhibition of Streptococcus faecium, Lactobacillus casei, and L1210 cells in culture by the target compounds was modest. They were also weak inhibitors of thymidylate synthase, dihydrofolate reductase, glycinamide-ribonucleotide transformylase, and aminoimidazolecarboxamide ribonucleotide transformylase. In contrast, 8-deazafolate showed moderate inhibition of aminoimidazolecarboxamide ribonucleotide transformylase, suggesting that inhibition of this enzyme may be related to its cytotoxic action. Tetrahydro-8-deazahomofolate showed low substrate activity with thymidylate synthase.

Synthesis and antifolate properties of 10-alkyl-8,10-dideazaminopterins.

The synthesis of 10-alkyl analogues of the potent antitumor agent 8,10-dideazaminopterin is described. Alkylation of appropriate alpha-alkyl homoterephthalate esters with 2,4-diamino-6-(bromomethyl)-8-deazapteridine afforded 10-alkyl-10-carboxy-4-amino-4-deoxy-8,10-dideazapteroic acid diesters. Ester cleavage and decarboxylation at C-10 were accomplished by heating with sodium cyanide in Me2SO at 170-180 degrees C to afford the 2,4-diamino-10-alkyl-8,10-dideazapteroic acids. The acids were coupled with diethyl glutamate, followed by saponification, to give the 10-alkyl-8,10-dideazaminopterins. The compounds were potent inhibitors of growth in folate-dependent bacteria, Streptococcus faecium and Lactobacillus casei. The 10-methyl and 10-ethyl analogues gave the highest percent increases in life span for mice infected with L1210 leukemia with ILS values of +203 and +235%, respectively.

Analogues of methotrexate in rheumatoid arthritis. 1. Effects of 10-deazaaminopterin analogues on type II collagen-induced arthritis in mice.

Carbonation of the dianions (LDA) of 5-methylthiophene-2-carboxylic, 2-methylpyridine-5-carboxylic, and 3-methylpyridine-6-carboxylic acids provided the respective carboxy heteroarylacetic acids. The crude diacids were directly esterified in MeOH-HCl to afford the diesters. Alkylation of the sodio anions with ethyl iodide yielded the appropriate alpha-ethyl diesters. The anions of the various diester substrates were then alkylated by 2,4-diamino-6-(bromomethyl)-pteridine followed by ester saponification at room temperature to afford the respective 2,4-diamino-4-deoxy-10-carboxy-10-deazapteroic acids. The 10-carboxyl group was readily decarboxylated by heating in DMSO at temperatures of 110-135 degrees C to give the diamino 10-deaza heteropteroic acid intermediates. Coupling with diethyl L-glutamate followed by ester hydrolysis afforded the target aminopterins. The analogues were evaluated for antiinflammatory effect in the mouse type II collagen model. The thiophene analogue of 10-ethyl-10-deazaaminopterin was found to be an effective inhibitor in terms of reduced visual evidence of inflammation and swelling as determined by caliper measurement.

Analogues of methotrexate in rheumatoid arthritis. 2. Effects of 5-deazaaminopterin, 5,10-dideazaaminopterin, and analogues on type II collagen-induced arthritis in mice.

Twenty-six compounds derived from the 5-deaza- and 5,10-dideazaaminopterin series of aminopterin analogues were evaluated for antiarthritic activity in the mouse type II collagen model. New compounds in the 5-deaza series were prepared by alkylation of an appropriate N-substituted (4-aminobenzoyl)-L-glutamic acid dialkyl ester or N-(5-amino-2-thenoyl)-L-glutamate diester with a 2,4-diamino-5-alkyl-6-(bromomethyl)-5-deazapteridine. The resultant 5-deazaaminopterin diesters were saponified to provide the target 5-deaza analogues. 5,10-Dideazaaminopterins were synthesized by similar alkylation of the carbanions of appropriate 4-carboxyphenylacetic, (5-carboxy-2-thienyl)acetic, or (5-carboxy-2-pyridyl)acetic acid dimethyl esters. The diesters of the 2,4-diamino-4-deoxy-10-carboxy-5,10-dideazapteroic acid types so obtained were saponified and then readily decarboxylated by heating in Me2SO solution to provide the 2,4-diamino-5,10-dideazapteroic acid-type intermediates. Peptide coupling with diethyl L-glutamate followed by ester hydrolysis at room temperature afforded the new 5,10-dideazaaminopterin analogues. 5-Deazaaminopterins bearing an alkyl substituent at the 5-position were generally quite effective as antiinflammatory agents. Thus 5-propyl-5-deazaaminopterin, 5-methyl-10-propargyl-5-deazaaminopterin, 5-methyl-10-allyl-5-deazaaminopterin, 5-ethyl-5-deazamethotrexate, and 2,5-disubstituted thiophene analogue of 5-methyl-5-deazaaminopterin showed potencies greater than methotrexate by intraperitoneal or oral administration and were active over a considerably broader dose range. Useful activity in the 5,10-dideaza series was only observed for 5,10-dideazaaminopterin and its 10-methyl analogue. Alkyl substitution at C-5 or C-10 was generally detrimental to antiinflammatory activity in this series.

Analgesics. 1. Synthesis and analgesic properties of N-sec-alkyl- and N-tert-alkylnormorphines.

A series of N-sec- and N-tert-alkylnormorphines was synthesized and evaluated for analgesic potency, antagonist activity, and opiate receptor binding. Computer-assisted conformational analysis profiles were utilized to assist in the selection of compounds for synthesis and correlation of receptor events with in vivo observations. N-tert-Alkylnormorphines 5a-c were devoid of agonist activity; however, some sec-alkyl analogues showed interesting mixed agonist-antagnoist actions. N-sec-Butyl- and N-(alpha-methylally)normorphine were separated into R and S isomers, which exhibited quantitative pharmacological differences. The N-sec-butyl S isomer 10a showed analgesia approximating morphine with nalorphine-like antagonist activity. Preliminary testing indicates only slight evidence for physical dependence with this compound.

Synthesis and biological activity of resolved C-10 diastereomers of 10-methyl- and 10-ethyl-10-deazaminopterin.

Synthesis and evaluation of the antitumor drugs 10-methyl- and 10-ethyl-10-deazaminopterin (15a,b) were previously reported for the diastereomeric mixtures, lacking resolution at the C-10 position. In order to assess biological properties of the individual diastereomers, the C-10 isomers of 4-amino-4-deoxy-10-methyl- and 10-ethyl-10-deazapteroic acids (13a,b) were prepared by total synthesis. Coupling with L-glutamate afforded the appropriate diastereomers of the title compounds. Biochemical, transport, and cell growth inhibitory properties in L1210 cells and folate-dependent bacteria were measured. Differences were generally less than 2-fold between diastereomeric pairs, but a factor of 3 was noted for d,L-15b vs. l,L-15b in inhibition of DHFR from L1210 cells and in cytotoxicity toward L1210 cells. An in vivo comparison of the isomers of 15b with racemic compound against L1210 in mice did not show a significant efficacy difference (ILS) among the compounds. However, d,L-15b showed an acute toxicity about 2.5 times that of l,L-15b.

Stabilized analogs of thymopentin. 2. 1,2- and 3,4-ketomethylene pseudopeptides.

In this second paper in a series of three studies of stable analogs of thymopentin (Arg1-Lys2-Asp3-Val4-Tyr5), the synthesis of analogs stabilized at peptide bonds 1,2 and 3,4 via insertion of ketomethylene units is described. A tris(carbobenzyloxy)arginyl(k)norleucine pseudopeptide was synthesized and coupled to Asp-Val-Phe-resin units followed by HF cleavage to prepare Arg(k)Nle-Asp-Val-Phe analogs. Preparation of N-BOC Asp(k)Val and N-BOC Asp(k)Ala units followed by coupling to Phe- or Tyr-resin units provided resin-bound pseudotripeptide substrates for attachment of various arginyl dipeptides. Cleavage from the resin afforded 3,4-ketomethylene-stabilized pseudopeptide analogs of thymopentin. The Arg-Lys-Asp(k)Val-Phe and Arg-Lys-Asp(k)Val-Tyr analogs were more strongly bound to CEM cells than thymopentin itself. There was significant enhancement of stability in serum for the analogs, especially those containing Arg(k)Nle or Arg-NMeLys moieties at the 1,2-peptide bond.