Reactions of {4-[Bis(2-chloroethyl)amino]phenyl}acetic Acid (Phenylacetic Acid Mustard) with 2′-Deoxyribonucleosides

Phenylacetic acid mustard (PAM; 2), a major metabolite of the anticancer agent chlorambucil (CLB; 1), was allowed to react with 2'-deoxyadenosine (dA), 2'-deoxyguanosine (dG), 2'-deoxycytidine (dC), 2'-deoxy-5-methylcytidine (dMeC), and thymidine (T) at physiological pH (cacodylic acid, 50% base). The reactions were followed by HPLC and analyzed by HPLC/MS and/or (1)H-NMR techniques. Although the predominant reaction observed was hydrolysis of PAM, 2 also reacted with various heteroatoms of the nucleosides to give a series of products: compounds 5-31. PAM (2) was found to be hydrolytically slightly more stable than CLB (1). The principal reaction sites of 2 with dA, dG, and with all pyrimidine nucleosides were N(1), N(7), and N(3), resp. Also, several other adducts were detected and characterized. There was no significant difference in the reactivity of 1 and 2 with dG, dA or T, but the N(3) dC-PAM adduct was deaminated easier than the corresponding CLB derivative. The role of PAM-2'-deoxyribonucleoside adducts on the cytotoxic and mutagenic properties of CLB (1) is discussed.

Metabolite changes in HT-29 xenograft tumors following HIF-1alpha inhibition with PX-478 as studied by MR spectroscopy in vivo and ex vivo

The hypoxia-inducible transcription factor (HIF-1alpha) plays a central role in tumor development. PX-478 is an experimental anti-cancer drug known to inhibit HIF-1alpha in experimental tumors. The purpose of this study was to identify MRS-visible metabolic biomarkers for PX-478 response prior to phase I/II clinical trials. Single-voxel in vivo localized (1)H spectra were obtained from HT-29 tumor xenografts prior and up to 24 h after treatment with a single dose of PX-478. Profiles of water-soluble and lipophilic metabolites were also examined ex vivo with both (1)H and (31)P spectroscopy for peak identification and to interrogate the underlying biochemistry of the response. The total choline (tCho) resonance was significantly decreased in vivo 12 and 24 h following treatment with PX-478 and this was confirmed with high-resolution (1)H and (31)P MRS. In non-aqueous extracts, significant reductions in cardiolipin, PtdEtn (phosphatidylethanolamine) and PtdI (phosphatidylinositol) were seen in response to PX-478. Although there were trends to a decrease in lactate (and lipid) resonances in vivo and ex vivo, these changes were not significant. This is in contrast to inhibition of in vitro glucose consumption and lactate production by PX-478 in HT-29 cells. The significant and robust change in tCho has identified this as a potential (1)H MRS-visible biomarker for drug response in vivo while high-resolution spectroscopy indicated that GPC, PC, myoI, PE, GPE, CL, PtdEtn and PtdI are potential ex vivo response biomarkers.

Novel fluorinated prodrugs for activation by carboxypeptidase G2 showing good in vivo antitumor activity in gene-directed enzyme prodrug therapy

Sixteen novel polyfluorinated benzoic acid mustards have been synthesized for use in gene-directed enzyme prodrug therapy (GDEPT). Eight of these were benzoic acid L-glutamate mustards for evaluation as prodrugs and the other eight were the active drugs formed by the action of the bacterial enzyme carboxypeptidase G2 (CPG2). All of the di- and trifluorinated prodrugs were efficiently cleaved by the enzyme. In contrast, the tetrafluorinated prodrugs were found to be competitive inhibitors of CPG2, the first such inhibitors to have been described. The di- and trifluorinated prodrugs were differentially cytotoxic to human breast carcinoma cells (MDA MB 361) expressing CPG2, compared to control cells that did not express the enzyme. The difluorinated prodrug {4-[bis(2-bromoethyl)amino]-3,5-difluorobenzoyl}-L-glutamic acid and its iodoethylamino analogue were effective substrates for the enzyme and showed excellent therapeutic activity in CPG2-expressing MDA MB 361 xenografts, either curing or greatly inhibiting tumor growth and extending the life of the animals.

Distamycin A as Stem of DNA Minor Groove Alkylating Agents

Analogues of naturally occurring antitumor agents, such as distamycin A, which bind in the minor groove of DNA, represent a new class of anticancer compounds currently under investigation. Distamycin A has driven researcher's attention not only for their biological activity, but also for its non intercalative binding to the minor groove of double-stranded B-DNA, where it forms strong reversible complex preferentially at the nucleotide sequences consisting of 4-5 adjacent AT base pairs. The pyrrole-amide skeleton of distamycin A has been also used as DNA sequence selective vehicles for the delivery of alkylating functions to DNA targets, leading to a sharp increase of its cytotoxicity, in comparison to that, very weak, of distamycin itself. In the last few years, several hybrid compounds, in which known antitumor derivatives or simple active moieties of known antitumor agents have been tethered to distamycin frames, have been designed, synthesized and tested. Several efforts have been made to modify DNA sequence selectivity and stability of the distamycin and the structural modifications have been based on replacement of pyrrole by other heterocycles and/or benzoheterocycles obtaining a novel class of minor groove binding molecules called lexitropsins. The role of the amidino moiety, by means of the substitution with various groups, which includes ionizable, acid or basic, and non-ionizable groups, has been also studied. The synthesis of a hybrid deriving among the combination of the distamycin A and naturally occurring alkylating agent has been also reported. Several classes of distamycin derivatives that have been reported in the published literature have been described in this review article.

Interaction of oligonucleotides with cellular proteins

Oligonucleotides (ODNs) conjugated to 4-[(N-2-chloroethyl-N-methyl)amino] benzylamine were used to investigate ODN-binding proteins in cells of different origin. The data obtained demonstrate that 68, 46, 38 and 28 kDa ODN-binding proteins are universal for tested cell lines.

Rapid determination of the anti-cancer drug chlorambucil (Leukeran) and its phenyl acetic acid mustard metabolite in human serum and plasma by automated solid-phase extraction and liquid chromatography-tandem mass spectrometry

A bioanalytical method for the determination of the anticancer drug chlorambucil (Leukeran) and its phenyl acetic acid mustard metabolite in human serum and plasma is described. Automated solid-phase extraction of the analytes is carried out with C18 sorbent packed in a 96 well format microtitre plate using a robotic sample processor. The extracts are analysed by isocratic reversed-phase liquid chromatography using pneumatically and thermally assisted electrospray ionisation (TurboIonspray) with selected reaction monitoring. The method is specific and sensitive, with a range of 4-800 ng/ml in human serum and plasma for both parent drug and metabolite (sample volume 200 microl). The method is accurate and precise with intra-assay and inter-assay precision (C.V.) of <15% and bias <15% for both analytes. The automated extraction procedure is significantly faster than manual sample pre-treatment methods, a batch of 96 samples is extracted in 50 min allowing for faster sample turnaround. The method has been used to provide pharmacokinetic support to biocomparability studies of Leukeran following single doses of oral tablet formulations.

DNA minor groove targeted alkylating agents based on bisbenzimidazole carriers: synthesis, cytotoxicity and sequence-specificity of DNA alkylation

A series of bisbenzimidazoles bearing a variety of alkylating agents [ortho- and meta-mustards, imidazolebis(hydroxymethyl), imidazolebis(methylcarbamate) and pyrrolebis(hydroxymethyl)], appended by a propyl linker chain, were prepared and investigated for sequence-specificity of DNA alkylation and their cytotoxicity. Previous work has shown that, for para-aniline mustards, a propyl linker is optimal for cytotoxicity. Alkaline cleavage assays using a variety of different labelled oligonucleotides showed that the preferred sequences for adenine alkylation were 5'-TTTANANAANN and 5'-ATTANANAANN (underlined bases show the drug alkylation sites), with AT-rich sequences required on both the 5' and 3' sides of the alkylated adenine. The different aniline mustards showed little variation in alkylation pattern and similar efficiencies of DNA cross-link formation despite the changes in orientation and positioning of the mustard, suggesting that the propyl linker has some flexibility. The imidazole- and pyrrolebis(hydroxymethyl) alkylators showed no DNA strand cleavage following base treatment, indicating that no guanine or adenine N3 or N7 adducts were formed. Using the PCR-based polymerase stop assay, these alkylators showed PCR blocks at 5'-C*G sites (the * nucleotide indicates the blocked site), particularly at 5'-TAC*GA 5'-AGC*GGA, and 5'-AGCC*GGT sequences, caused by guanine 2-NH2 lesions on the opposite strand. Only the (more reactive) imidazolebis(methylcarbamoyl) and pyrrolebis(hydroxymethyl) alkylators demonstrated interstrand cross-linking ability. All of the bifunctional mustards showed large (approximately 100-fold) increases in cytotoxicity over chlorambucil, with the corresponding monofunctional mustards being 20- to 60-fold less cytotoxic. These results suggest that in the mustards the propyl linker provides sufficient flexibility to achieve delivery of the alkylator to favoured (adenine N3) sites in the minor groove, regardless of its exact geometry with respect to the bisbenzimidazole carrier. The 'targeted' bisbenzimidazole bis(hydroxymethyl)pyrrole- and imidazole analogues showed very similar patterns of alkylation to the corresponding 'untargeted' compounds, with little evidence of additional selectivity imposed by this AT-preferring carrier.

Synthesis of self-immolative glucuronide-based prodrugs of a phenol mustard

The design and synthesis of the mustard pro-prodrugs which can be used in ADEPT is reported. Prodrugs 1 and 2 include a glucuronide group which is connected to the drug via an aromatic and/or aliphatic bis-carbamate spacer. The design of these new prodrugs takes advantage of a spontaneous 1,6-elimination and/or an intramolecular cyclization reaction after enzymatic cleavage. Thus, enzymatic-catalyzed hydrolysis of the glucuronyl moiety of 1 by Escherichia coli beta-glucuronidase results in the liberation of the parent mustard drug 20 with formation of CO2, 2-nitro-4-hydroxymethylphenol 19 and dimethylimidazolidinone 21. Surprisingly, prodrug 2 was not cleaved under the same conditions. According to in vitro experiments, prodrugs 1 and 2 were approximately 50- and 80-fold less cytotoxic than the parent drug and, when treated with beta-glucuronidase, the level of cytotoxic activity of 1 became comparable to that of the drug. Stability of 1 in phosphate buffer was satisfactory. These results demonstrate that 1 is a prodrug that can be specifically activated to release the cytotoxic agent.

Prodrug strategies based on intramolecular cyclization reactions

Several new prodrug systems for amines, alcohols, and peptides are reviewed. The design of these new prodrug systems takes advantage of several facile intramolecular cyclization reactions, that permit separate manipulation of the release kinetics independent of the structural features of the drug moiety. Such systems can be used for the preparation of esterase-, phosphatase-, and redox-sensitive prodrugs of amines and alcohols and esterase-sensitive cyclic prodrugs of peptides and peptide mimetics.

Design, synthesis, and evaluation of latent alkylating agents activated by glutathione S-transferase

In search of compounds with improved specificity for targeting the important cancer-associated P1-1 glutathione S-transferase (GST) isozyme, new analogs 4 and 5 of the previously reported glutathione S-transferase (GST)-activated latent alkylating agent gamma-glutamyl-alpha-amino-beta-[[[2-[[bis[bis(2-chloroethyl)amino]ph osp horyl]oxy]ethyl]sulfonyl]propionyl]-(R)-(-)-phenylglycine (3) have been designed, synthesized, and evaluated. One of the diastereomers of 4 exhibited good selectivity for GST P1-1. The tetrabromo analog 5 of the tetrachloro compound 3 maintained its specificity and was found to be more readily activated by GSTs than 3. The GST activation concept was further broadened through design, synthesis, and evaluation of a novel latent urethane mustard 8 and its diethyl ester 9. Interestingly, 8 showed very good specificity for P1-1 GST. Cell culture studies were carried out on 4, 5, 8, and 9 using cell lines engineered to have varying levels of GST P1-1 isozyme. New analogs 4 and 5 exhibited increased toxicity to cell lines with overexpressed GST P1-1 isozyme. The urethane mustard 8 and its diethyl ester 9 were found to be not as toxic. However, they too exhibited more toxicity to a cell line engineered to have elevated P1-1 levels, which was in agreement with the observed in vitro specificity of 8 for P1-1 GST isozyme. Mechanistic studies on alkaline as well as enzyme-catalyzed decomposition of latent mustard 3 provided experimental proof for the hypothesis that 3 breaks down into an active phosphoramidate mustard and a reactive vinyl sulfone. The alkylating nature of the decomposition products was further demonstrated by trapping those transient species as relatively stable diethyldithiocarbamic acid adducts. These results substantially extend previous efforts to develop drugs targeting GST and provide a paradigm for development of other latent drugs.

Design, synthesis and biological evaluation of benzoic acid mustard derivatives of imidazole-containing and C-terminal carboxamide analogues of distamycin

The synthesis, DNA binding and biological evaluation of two benzoic acid mustard derivatives of imidazole-containing analogues of distamycin in which the C-terminus is modified to contain a terminal carboxamide are described. The apparent DNA binding constants of compounds 5 and 6 were determined using an ethidium displacement assay, and the results showed that they do not have the AT sequence selectivity of distamycin and they show an acceptance for GC base pairs. Based on their pronounced binding to T4 DNA the data suggest that they bind to the minor groove of DNA. The cytotoxicities of compounds 5 and 6 in human chronic myeloid leukemia cells were determined using a MTT assay, and their IC50 values were 27 and 16 microM, respectively, and higher than the corresponding non-terminal carboxamide-containing analogues 3 and 4. Both compounds were however markedly more active than the non-targeted mustard BAM [N,N-bis (-2-chloroethyl)-4-aminobenzoic acid]. In the NCI panel of cell lines 5 gave a distinctly different pattern of tumor selectivity from 6. While these compounds were shown to alkylate DNA using a CD alkylation assay (35 +/- 10% for 5 and 85 +/- 10% for 6), they produced interstrand crosslinks poorly, even at 100 microM drug concentrations. Based on preliminary data from a polymerase stop assay compounds 3-6 gave different patterns of sequence selection monoalkylation which may contribute to their differing biological activities.

[Oligonucleotide-binding proteins of Drosophila: tissue specificity]

A reaction of native Drosophila proteins with an alkylating oligonucleotide derivative bearing 4-[(N-2-chlorethyl-N-methyl)amino]benzylamine at the 5' terminal phosphate has been investigated. It was found, that the reagent alkylates a few proteins (90, 50, 44, 39, 32 kDa). The modification was organ specific. The labeled 39 kDa protein is present in the ovaries only, while the modified 32 kDa protein is found only in the bulbus.

The use of chemical weapons. Conducting an investigation using survey epidemiology

The use of chemical weapons in conflict represents a breach of international law as well as a grave violation of human rights. Investigating allegations of their use often is difficult. A basic tool is the survey interview. Experience has shown that a rigorous epidemiologic approach should be taken. A primary emphasis should be designing the study so that consistency of responses can be analyzed to judge the validity of the testimony. Only when the testimony can withstand this scrutiny is it possible to surmise the possible identity of agents employed. Securing samples of the putative agent is of obvious importance. Two recent investigations are discussed herein, one conducted by US Army medical researchers on allegations of chemical weapons use against the Hmong in Laos and another mounted by us on allegations of poison-gas attack against the Iraqi Kurds.

[Chemical transformation of radioactive 4-(N-2-chloroethyl-N-methylamino)benzyl-5'-[32P]phosphamides of oligodeoxyribonucleotides during in vivo experiments]

The practical use of reactive oligonucleotide derivatives for complementarily addressed modification of nucleic acids in vivo includes several steps, at which side chemical reactions resulting in a decrease of the modification efficiency may take place. Chemical reactions of 4-(N-2-chloroethyl-N-methylamino)benzyl-5'-[32P]phosphamides of oligodeoxyribonucleotides were studied in vivo. The intermolecular self-alkylation at the reactive residue of the alkylating derivative was found in the precipitate of its lithium salt under acetone at-20 degrees C. The effects of pH, buffer solutions, salts, temperature, phenol, cell culture suspensions, tissue homogenates, etc., on the stability of the derivatives were studied. A sufficient cleavage of the phosphamide bond was observed at pH less than 3. In fresh liver homogenates the nucleolytic degradation of the oligonucleotide part of the reagent was shown to occur. After intraperitoneal injection of mice with radioactive alkylating derivatives up to 50% of the reagent was included into the blood biopolymers within one hour. The covalently linked to the biopolymers oligonucleotide appeared to be highly degraded thereby.

Analysis of thiodiglycol in urine of victims of an alleged attack with mustard gas

A procedure for the semi-quantitative determination of thiodiglycol, a metabolite of the vesicant mustard gas, in urine has been developed. Thiodiglycol was converted into mustard gas using concentrated HCl at temperatures close to 100 degrees C. The headspace of the solution containing mustard gas, was trapped on an adsorption tube filled with Tenax-GC which was subsequently analyzed by gas chromatography/mass spectrometry. Using 10 mL of urine, a detection limit of a few ng/mL of thiodiglycol was achieved. The procedure was applied to urine samples obtained from Iranian patients who were the alleged victims of an attack by chemical warfare agents (probably mustard gas). A number of control samples were investigated as well. Thiodiglycol was found in the urine of the Iranian patients in concentrations varying between 3 and 140 ng/mL. However, the detection of thiodiglycol in concentrations up to 55 ng/mL in control samples excluded the unambiguous verification of the use of mustard gas against the Iranian patients.

The hydrolysis and alkylation activities of S-(2-haloethyl)-L-cysteine analogs--evidence for extended half-life

A series of S-(2-haloethyl)-L-cysteine derivatives, which are analogs of the proposed glutathione half-mustard metabolites of dihaloethanes, were synthesized and studied with respect to their hydrolysis and alkylation rates in aqueous solution. The trend of relative hydrolysis rates, Br greater than Cl much greater than F, paralleled their respective leaving group abilities; however, a dramatic rate increase was seen at pH 8 versus pH's 6 or 4. Hydrolysis of S-(2-chloroethyl)-L-cysteine analogs, where the ionizable groups were blocked (carboxyl esterified and/or N-acetylated), revealed that the amine moiety was responsible for the increased hydrolysis of mustard gas (beta, beta'-dichlorodiethyl sulfide) gave similar results with S-(2-chloroethyl)-L-cysteine, a finding which is consistent with the reaction intermediate being a highly charged species. The alkylation rates with 4-(p-nitrobenzyl)-pyridine were not affected by blocking the ionizable groups. A mechanism of internal cyclization is proposed to explain the accelerated alkaline hydrolysis rates noted with S-(2-haloethyl)-L-cysteines but not with the N-acetylated analogs (mercapturic acids). This scheme proposes the formation of 3-(thiomorpholine)-carboxylic acid as an alternative pathway to the generally accepted hydrolysis reaction. This compound and not S-(2-hydroxyethyl)-L-cysteine was the identified product following pH 10 hydrolysis. Increased hydrolysis half-time of amine-blocked cysteine analogs versus parent cysteine analogs may exist with S-(2-haloethyl)-glutathione derivatives which may explain the substantial nucleic acid alkylation seen with S-(2-haloethyl) derivatives of glutathione.

PCNU: a new nitrosourea in clinical oncology

PCNU is a new nitrosourea compound which has recently entered clinical trials. Preclinically it has been found to be effective against a variety of tumor models. Biochemically, PCNU was found to have optimal lipophilic, alkylating, and carbamoylating properties as compared to other nitrosourea agents. PCNU is able to diffuse into the CSF as demonstrated by pharmacokinetic studies. Clinical phase I studies indicate that the main toxicity is myelosuppression; nausea and vomiting were less frequently observed with PCNU than with other nitrosourea compounds. Human antitumor activity has been reported in a number of tumors, but most consistently in brain tumors. Phase II studies are now underway to confirm the antitumor activity of PCNU.

Phase I trial and clinical pharmacology of 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea

1-(2-Chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea (NSC 95466) is a lipid-soluble nitrosourea that is presently undergoing clinical evaluation. In this Phase I study, the toxicity of this drug was examined after administration of the drug to cancer patients on 3 successive days every 6 to 8 weeks. Clinical pharmacology was studied using 1-[chloroethyl-14C](2-chloroethyl)-3-(2,6-dioxo-3-piperidyl)-1-nitrosourea. The dose-limiting toxicity was myelosuppression. The maximal tolerated dose was 105 mg/sq m, which produced a median platelet nadir of 40,000/microliter on Day 32 and a median white blood cell count nadir of 2200/microliter on Day 42. Progressive anemia was also observed. There was no evidence of acute or chronic hepatic, renal, or pulmonary damage. One patient with a metastatic hypernephroma exhibited a partial clinical remission. Plasma disappearance of the drug following bolus administration was biphasic, with an initial half-life of 18 to 25 min and a second half-life of 9 hr. Clearance of intact drug coincided largely with the initial disappearance phase of total radioactivity. Entry of radioactivity into the cerebrospinal fluid was observed. Approximately 35% of plasma radioactivity was protein bound, the major binder being albumin. Drug excretion was predominantly renal, and biliary elimination was only minor.

Affinity of estradiol mustard for estrogen receptors and its enzymatic degradation in uterine and breast cancer cytosols

Estradiol mustard (EM) is the 3,17beta-diester of estradiol-17beta (E2) with the nitrogen mustard derivative chlorphenacyl. The ability of EM to bind to cytoplasmic estrogen receptors was tested by inhibition of the binding of 3H-E2 to rat uterine cytosol at 18 degrees C and 30 degrees C. At both temperatures an inhibition curve was observed in the presence of a large excess of drug, suggesting that the latter has a very weak binding affinity (100,000 times lower than E2). Incubation of uterine cytosol with increasing amounts of 3H-E2 in the presence and absence of an excess of EM indicated that the drug interacted with the receptors at the same sites as E2 (competitive inhibition). Preincubation of uterine cytosol at 18 degrees C with EM induced a progressive reduction of 3H-E2 binding capacity. This reduction also occurred, although to a lesser extent, when long-term incubation of the cytosol with EM was performed in the presence of labelled E2 from the start. The process was faster at 18 degrees C than at 4 degrees C and did not occur with EM preincubated in homogenization buffer. Exchange assays by 3H-E2 of uterine receptors preincubated with labelled E2 and excess EM indicated that the drug-induced inhibition of binding capacity was reversible and produced no apparent alteration of the receptors. Furthermore, the rate of exchange was similar to that observed with receptors previously filled with unlabelled E2. In 9 "receptor-positive" cytosols from human breast cancers, time-course study of the binding of 3H-E2 in the presence of excess of EM yielded similar results as those obtained with rat uterine cytosol. These results show that EM has a very low binding affinity for the extrogen receptors and that it is metabolized into one or several compounds of higher binding affinity. They suggest that EM is probably not significantly concentrated by the estrogen target tissues such as mammary cancers. Therefore, the drug is unlikely to be very valuable in the treatment of breast cancer through a specific mechanism involving concentration by the estrogen receptors.

Muscarinic acetylcholine receptor from rat brain. Partial purification and characterization

A protein capable of binding atropine and (3H)propylbenzilylcholine mustard was solubilized and purified (200-fold) from rat brain. Pronase and trypsin, but not phospholipases, diminished the binding capacity of the solubilized receptor. The molecular weight of the salt-solubilized receptor as determined by gel filtration in the absence of detergents is 30,000. The purified protein showed specificity of binding toward muscarinic ligands. the high and low affinity dissociation constants of the receptor.atropine complex are 0.3 nM and 0.15 muM. Binding of atropine is pH-dependent with an optimum at 7.1. Ca2+ influences the binding of atropine and maximal binding occurs at 0.5 mM Ca2+. The subcellular distribution of the receptor was also examined.

Muscarinic receptors in rat intestinal muscle: comparison with the guinea pig

The properties of muscarinic receptors in rat intestinal muscle have been investigated by examining the binding of 3H-propylbenzilylcholine mustard, 3H-PrBCM, a specific and virtually irreversible muscarinic antagonist. It is shown that the properties of these muscarinic receptore correspond in general closely to those observed in the guinea pig, in particular the binding curve inferred for carbachol shows the same apparent negative cooperativity, but there is a notable difference in the rate of binding 3H-PrBCM.

The properties of muscarinic receptors in mammalian cerebral cortex

1 The reaction of tritiated propylbenzilylcholine mustard ([(3)H]-PrBCM; N-2'-chloroethyl-N-[2'',3''-(3)H(2)] -propyl-2-aminoethylbenzilate) with homogenates of mammalian brain has been studied.2 The uptake can be divided into an atropine-sensitive component of fixed capacity (380 pmol/g protein in the rat) and an atropine-insensitive part.3 The atropine-sensitive portion is identified as muscarinic receptor by its insensitivity to nicotinic antagonists and anticholinesterases and its sensitivity to a range of muscarinic antagonists.4 The uptake of [(3)H]-PrBCM is also inhibited by muscarinic agonists and there is reasonable quantitative agreement between the affinities of agonists estimated in this way and in intact tissues by physiological responses.5 The fraction of [(3)H]-PrBCM uptake inhibited by muscarinic antagonists and agonists is the same.6 The amount of receptor found in six mammalian species was inversely related to the size of the brain, but the rates of alkylation and the sensitivity to atropine were not dissimilar.

The binding of (3H)-propylbenzilycholine mustard by longitudinal muscle strips from guinea-pig small intestine

1 The synthesis of tritium labelled propylbenzilylcholine mustard ([(3)H]-PrBCM; N-2'-chloroethyl-N-[2'', 3''-(3)H(2)] propyl-2-aminoethyl benzilate) is described.2 The uptake by muscle strips was measured and shown to be considerably increased by previous immersion of the muscle in distilled water.3 A considerable part of the uptake is inhibited selectively by atropine, but not by nicotinic antagonists. A number of muscarinic agonists also inhibit uptake and their apparent affinity constants have been determined.4 The uptake by atropine-sensitive sites is temperature-insensitive, whereas the other sites are temperature-sensitive. Recovery is highly temperature-sensitive and there is good agreement between recovery of sensitivity to agonists and loss of radioactivity from the muscle.

The number of muscarinic receptors in chick amnion muscle

1. [(3)H]-Propyl benzilylcholine mustard ([(3)H]-PrBCM) an irreversible specific antagonist of muscarinic acetylcholine receptors, has been used to label the muscarinic receptors in the aneuronal muscle of isolated 11 day amniotic membranes of chick embryos.2. This muscle was found to have about 9 fmol/mg dry weight of receptor material. This is only 1-2% of the amount found in intestinal muscle.3. Pharmacological studies with isolated amniotic membranes using PrBCM show that labelling is to functional receptors and that the receptor reserve is small.4. The significance of the difference in receptor reserve in innervated and aneuronal plain muscle is discussed.

Access of drugs to tumours of the large intestine

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The penetration of cytotoxins into malignant tumours

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