Chemotherapy with paclitaxel and gemcitabine in progressive medullary and thyroid carcinoma of the follicular epithelium.

Nine patients (mean age 53) with metastasizing, progressive, medullary (MTC), thyroid carcinoma and progressive, nonradioiodine accumulating thyroid carcinoma of the follicular epithelium (follicular carcinoma, FTC and papillary carcinoma, PTC) were treated with a combination of paclitaxel and gemcitabine between 2004 and 2006. Tumors were histologically classified as follicular in 5 patients (56%), as papillary in 2 patients (22%), and medullary in 2 patients (22%). Paclitaxel (90-100 mg/m (2)) and gemcitabine (1,000 mg/m (2)) were applied for two, three, or 6 cycles every three weeks, depending on response and side effects. The effect of therapy was evaluated by radiographic imaging (CT images) and [(18)F]FDG-PET. All patients with papillary, follicular, or medullary thyroid carcinoma had continuous progression during restaging 14.8+/-8.8 weeks after initiation of chemotherapy, including one patient with stable disease after 3 cycles, but continuous progression after 6 cycles of chemotherapy. Paclitaxel and gemcitabine are not a valid chemotherapy option, in particular in patients with progressive, nonradioiodine-accumulating follicular thyroid carcinoma, who were already treated by other chemotherapeutic agents.

Weekly administration of sagopilone (ZK-EPO), a fully synthetic epothilone, in patients with refractory solid tumours: results of a phase I trial.

BACKGROUND: Epothilones are a novel class of microtubule-stabilising agents, and sagopilone is a fully synthetic epothilone that has shown marked in vivo and in vitro preclinical activity. METHODS: This phase I, open-label study investigated the maximum tolerated dose (MTD) and dose-limiting toxicities (DLTs) of weekly sagopilone. Twenty-three patients with malignancy resistant or refractory to standard treatment were enrolled into this study evaluating sagopilone doses from 0.6 to 7.0 mg m(-2). RESULTS: The incidence of drug-related haematological adverse events (AEs) was low, with two grade 3 events observed. Nonhaematological AEs were generally mild and reversible; increased gamma-GT was the only grade 4 event and grade 3 events comprised peripheral neuropathy (n=2), diarrhoea (n=1) and fatigue (n=1). Two grade 3 events were DLTs (diarrhoea and peripheral neuropathy at 7.0 mg m(-2)). The MTD of weekly sagopilone was therefore established as 5.3 mg m(-2). Stable disease was the best overall response (n=3). Microtubule bundle formation in peripheral blood mononuclear cells increased post-treatment, peaking after 1 h. Sagopilone disposition was similar across treatment courses and showed rapidly decreasing serum concentrations after infusion end and a long terminal disposition phase with no obvious accumulation in the serum, probably reflecting a fast uptake into tissues followed by a slow release. CONCLUSION: Weekly administration of sagopilone could represent an alternative to the 3-weekly administration currently evaluated in phase II trials.

C-reactive protein is a strong independent predictor of death in type 2 diabetes: association with multiple facets of the metabolic syndrome.

INTRODUCTION: It has been suggested that atherosclerotic vascular disease is a chronic inflammatory process. The aim of this study was to investigate the importance of C-reactive protein (CRP) as a cardiovascular risk marker and predictor of death, as well as its relation to other factors of the metabolic syndrome in a cohort of type 2 diabetic patients at high risk of severe macrovascular complications. MATERIAL AND METHODS: 592 patients, aged 55 to 74 years (311 men, 281 women), with signs and symptoms of circulation deficits were examined by duplex ultrasound for suspected cerebrovascular and peripheral arterial disease and followed over a period of 5 years. At baseline, 292 patients of the total group had type 2 diabetes (49.3%). Ischemic heart disease was present in 40.2%, internal carotid stenosis in 21.9% and peripheral arterial disease in 39.7% of the subjects. RESULTS: During the observation period, 104 patients had died, 72 (69.2%) due to cardiovascular causes. Non-fatal myocardial infarction occurred in 39 patients (7.4%), non-fatal stroke in 70 patients (13.3%) and amputations because of gangrene were unavoidable in 24 patients (4.6%). In Cox regression analysis, CRP was the strongest predictor of death and cardiovascular death in the total cohort (RR 3.7 [95% CI 1.86-7.50] and 5.4 [2.13-13.76]), as well as in the type 2 diabetic subgroup (RR 3.3 [1.27-8.70] and 5.4 [1.44-20.0]). In contrast neither the traditional cardiovascular risk factors nor the data of diabetic metabolic control were able to improve prediction. CRP was correlated positively with plasma levels of triglycerides (r=0.19, p=0.002), C-peptide (r=0.21, p=0.004), postprandial glucose (r=0.17, p=0.009), albuminuria (r=0.16, p=0.020), and inversely with HDL cholesterol (r=-0.20, p=0.002) in type 2 diabetic patients. CONCLUSIONS: CRP seems to be a better predictor of death and cardiovascular events than traditional risk factors or parameters of metabolic control in type 2 diabetic patients at high risk for cardiovascular endpoints. Additionally, CRP is associated with several facets of the metabolic syndrome.

Adenocarcinoma R-3327 of the Copenhagen rat as a suitable model for immunological studies of prostate cancer.

An experimental animal model for the study of host-prostatic tumor cell interactions has been described. R-3327, a line of prostatic adenocarcinoma of the Copenhagen rat, has been proven to be immunogenic to its syngeneic host as evidenced by two different in vitro cell-mediated immune assays. Specificity of the responses has been ascertained on the basis of absence of response: (a) of nonimmune lymphocytes to the R-3327 tumor antigen(s); (b) of R-3327 immune lymphocytes to several normal tissues including normal prostate; (c) of immune lymphocytes to unrelated squamous cell prostatic carcinoma of the Copenhagen rat. Furthermore, the presence of tumor has an effect in several nonspecific aspects of host response, inducing splenomegaly, heightened responses to nonspecific mitogens in lymphocyte transformation assay, and increased levels of killer cell action. Since there are many histological, biochemical, and functional analogies between this tumor line and human prostate carcinomas, this system appears to be suitable for immunological and possible immunotherapeutic studies of this type of neoplasia.

Cross-reactivity of a monoclonal antibody directed against an estrogen-induced protein in MCF-7 human breast cancer cells with murine Leydig cell tumor proteins.

An estrogen-responsive murine Leydig cell tumor (M5480A) was examined for the presence of cross-reactive proteins to a monoclonal antibody directed against a Mr 24,000 estrogen-regulated protein in human breast cancer cells. Human breast tumor biopsies were used as controls for the cytosol preparations, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and Western blot conditions used in these experiments. The estrogen-regulated Mr 24,000 protein was detected in sodium dodecyl sulfate-polyacrylamide gels of cytosols from four human breast tumor biopsies examined. Larger amounts of the Mr 24,000 protein were present in the two estrogen-progesterone receptor-positive tumor biopsies in comparison to the two estrogen-progesterone receptor-negative samples. In addition, the two receptor-positive samples demonstrated an additional, less intense immunoreactive band at Mr 21,000. Under identical conditions, the same monoclonal antibody bound to two major protein bands from sodium dodecyl sulfate-polyacrylamide gels of Leydig cell tumor cytosols at Mr 56,000 and Mr 86,000. Antibodies prepared from BALB/c mouse ascites fluid of animals bearing the parent myeloma cell line (P3X63NS1) exhibited no immunoreactivity against the human breast or Leydig cell tumor proteins. In light of the high degree of specificity which this monoclonal antibody exhibits, our results suggest that similar antigenic determinants may exist in these proteins from two distinct tumors.

5alpha-dihydrotestosterone-binding proteins and androgen sensitivity in prostatic cancers of Copenhangen rats.

A line (R-3327-A) of androgen-insensitive squamous cell carcinomas of the prostate of Copenhagen rats has been derived from an established line (R-3327) of androgen-responsive tumors R-3327 A tummors have a growth rate of approximately 10 times that of R-3327. The response of R=3327 A to hormonal environment has been determined by measuring the growth rate of the tumor transplanted to males, females, and castrated males. No great differences were observed. The metabolism of testosterone by tumore samples was studied following s.c. injections of the labeled steroid. The results show very little 5alpha-reduction of testosterone compared with that obtained in the prostate gland itself, as well as in the androgen-sensitive R-3327 tumors. A comparison of the presence of 17beta-hydroxy-5alpha-androstan-3-one-binding proteins in cytoplasmic extracts from both lines of tumors shows that the receptor proteins are present only in the androgen-sensitive R-3327 and not in the androgen-insensitive R-3327 A. The levels of the receptor proteins in R-3327 tumors are higher in tumors borne by male than by female animals, and castration of males decreases the amount of 17beta-hydroxy-5alpha-androstan-3-one binding.

Novel cellular determinants for reversal of multidrug resistance in cells expressing P170-glycoprotein.

The newly synthesized calcium channel blocker, Ro44-5912, significantly potentiates doxorubicin (Dox)-induced cytotoxicity at non-cytotoxic concentrations in Dox-resistant human ovarian cell line, A2780/DX5, overexpressing P170-glycoprotein (Pgp). Induction of DNA single- and double-strand breaks (ssbs and dsbs) was measured using alkaline elution and constant-field gel electrophoresis (CFGE) assays. The results indicate that potentiation of the cytotoxicity of Dox by Ro44-5912 was accompanied by significant increases in both, Dox-induced DNA ssbs and dsbs in the resistant cells. Pulsed-field gel electrophoresis (PFGE) analysis showed that Dox induced DNA fragments in the 50-800 kilobase (kb) and 0.8-5.7 megabase (Mb) ranges. The majority of the newly synthesized DNA fragments were in the 50-800 kb range. Ro44-5912 treatment resulted in significant potentiation of DNA fragmentation in the 50-800 kb range with a minor increase in 0.8-5.7 Mb DNA fragments, suggesting that the modulator functions by potentiating nascent DNA fragmentation in the resistant cells. Exposure to Dox with Ro44-5912 was associated with a prolonged blockage of cells in the S-phase. In contrast, exposure to Dox alone resulted in temporary blockage of cells in G2/M phase (approximately 24 h) followed by restoration of cell proliferation and normal DNA histograms at 48 h after 2 h drug exposure. Incorporation of BrdUrd by flow cytometric analysis was inhibited by Dox in the presence of Ro44-5912, showing that there is a block of DNA replication. An increased damage in newly synthesized DNA could concur with a blocked DNA replication. Moreover, slowing progression through the S-phase in cells exposed to Dox in combination with Ro44-5912 is accompanied by increased sensitivity of Dox poisons, indicating a correlation of specific S-phase perturbation with the reversal of Dox resistance by Ro44-5912 in cells expressing Pgp. The results suggest that drug-induced augmentation of nascent DNA fragmentation and specific cell-cycle perturbation are potentially important molecular determinants for reversal of multidrug resistance in addition to restoration of intracellular drug retention.

Potential activity of paclitaxel, vinorelbine and gemcitabine in anaplastic thyroid carcinoma.

Anaplastic thyroid carcinoma (ATC) has a rapidly fatal course in the mostly elderly patients with a median survival after diagnosis of 4-12 months. Activity of commonly used chemotherapy (doxorubicin) is low, thus more active compounds need to be introduced into the therapeutic concept of ATC. Recently, based on preclinical data Ain et al. conducted a clinical phase II study with paclitaxel 96 h infusion in ATC achieving a promising response rate of 53%. To further improve therapeutic options in ATC, we evaluated the activity of topotecan, oxaliplatin, vinorelbine, gemcitabine and paclitaxel in comparision to cisplatin and doxorubicin (1 and 96 h drug exposure) alone or in combination in the ATC cell lines SW1736 and 8505C. IC50 values were determined by the sulforhodamine B assay, potential clinical activity was estimated by relative antitumor activity (RAA) and drug interaction was analyzed using a parametric response surface approach (Greco model) of the Loewe additivity. Duration of drug effect was estimated by regrowth kinetics. We found paclitaxel, vinorelbine and gemcitabine active in ATC with RAA (1 h drug exposure) ranging from 86 to 454, 15 to 17 and 31 to 140, respectively. The activity of doxorubicin and cisplatin was moderate with RAA ranging from 1.4 to 2.2 and 0.2 to 0.3, respectively. Combined drug exposure of gemcitabine/paclitaxel and gemcitabine/vinorelbine was synergistic with a Loewe index > 0. However, these results did not reach statistical significance with p > 0.05. At clinically relevant drug concentrations paclitaxel, gemcitabine and vinorelbine but not oxaliplatin exerted a sustained growth inhibition after cessation of drug exposure for the complete assay time of 15 days. In conclusion, paclitaxel, gemcitabine and vinorelbine but not topotecan or oxaliplatin appeared to be active in anaplastic thyroid carcinoma based on RAA or growth delay at clinically relevant drug concentrations. Combinations of vinorelbine/gemcitabine and paclitaxel/gemcitabine exerted a trend to synergy. Thus, further evaluation of paclitaxel, vinorelbine and gemcitabine alone or in combination with ATC seems warranted.

Schedule-dependent antagonism of gemcitabine and cisplatin in human anaplastic thyroid cancer cell lines.

Anaplastic thyroid carcinoma (ATC) affects primarily elderly patients, with a median survival of 4-12 months after diagnosis. Presently, under clinical investigation the combination of cisplatin (CDDP) and gemcitabine (GEM) has promising activity in several of human tumor types. To develop new approaches for therapy of ATC, we evaluated the antineoplastic activity of GEM and CDDP alone (1-h and 24-h drug exposure) or in combination in the ATC cell lines SW1736, 8505C, C643, and HTh74. IC50 values were determined by the sulforhodamine B assay, activity was evaluated by the relative antitumor activity (RAA) and drug interaction assessed by isobologram analysis. GEM seemed to be active in ATC, with RAA ranging from 12-114 and CDDP only modestly active (RAA, 0.24-1.4). In four different drug schedules tested, the drug combination was additive when GEM preceded CDDP exposure (combination index, approximately 1), whereas when CDDP preceded GEM exposure the combination was significantly antagonistic (combination index, >1). In SW1736 and 8505C cells, we observed a strong S phase arrest and DNA synthesis inhibition 24 h after a 1-h exposure to an IC50 of CDDP. On the basis of molecular drug targets, cell cycle arrest points, and DNA synthesis inhibition, a model was developed to explain the interaction observed for the combination of GEM and CDDP. In conclusion, GEM shows promising cytostatic activity in ATC. Interaction of GEM and CDDP was schedule and dose dependent, favoring a regime in which GEM is followed by CDDP. Additionally, our model system suggests that DNA-synthesis inhibition and S phase arrest may be important determinants for the drug interaction between GEM and CDDP.

delta 5-3 beta-Hydroxysteroid dehydrogenase delta 4-5-isomerase activity and metabolism of dehydroepiandrosterone in rat preputial gland.

After intracardial injection of [1,2-3H]dehydroepiandrosterone ([3H]DHA) into female rats, [3H]DHA was found to accumulate and was metabolized in the preputial gland, but not in the diaphragm. The identified metabolites of [3H]DHA in the preputial gland were delta 4-androstenedione-3 alpha, 17 beta-diol. Cells were isolated from the preputial gland after treatment with trypsin and collagenase III, and centrifugation in Ficoll gradients. Activity of the enzyme complex delta 5-3 beta-hydroxysteroid dehydrogenase delta 4-5-isomerase (3 beta-HSD) responsible for transforming DHA into delta 4-androstenedione was found mainly in the 105,000 g pellet (microsomal fraction) of homogenates of the isolated cells. It used preferentially NAD over NADP as a coenzyme, with a pH optimum at 8.5. The apparent Km for DHA was 5.5 X 10(-5) M, and the Vmax was 1.72 nmol/min/mg microsomal protein. These findings indicate that DHA is preferentially taken up by the preputial gland where it undergoes metabolism to form more potent androgens, and suggest that DHA may have important androgenic influence on the preputial gland.

Efficacy of ciprofloxacin in stationary-phase bacteria in vivo.

The granuloma pouch model in mice infected with Escherichia coli or Pseudomonas aeruginosa was used to investigate the bactericidal effect of ciprofloxacin in vivo on bacteria in the stationary growth phase. Ciprofloxacin caused a rapid decline in the number of colony-forming units (cfu) of E. coli shortly after initiation of therapy (40 mg/kg, intraperitoneally). Ciprofloxacin was more effective than norfloxacin or pefloxacin and comparable in efficacy to ofloxacin. The drugs penetrated well into the pouch exudate, exceeding the minimal inhibitory concentrations (MICs) of the infecting organisms. The concentrations of pefloxacin or ofloxacin were higher than those of norfloxacin or ciprofloxacin. Ciprofloxacin also showed good killing effects in pouches infected with one strain of P. aeruginosa (ICB 7453, MIC of 0.06 micrograms/ml). However, with another P. aeruginosa strain (ICB 7933), which has a MIC of 0.5 micrograms/ml, killing of stationary cells in vivo was not very pronounced. Electron microscopic evaluation of the pouch exudate revealed that phagocytosed and non-phagocytosed E. coli cells were severely damaged in comparison with untreated control cells. The earliest ultrastructural changes could be observed 15 minutes after initiation of therapy. The results demonstrate that ciprofloxacin is effective in mice for the treatment of a local inflammatory abscess harboring a stationary population of E. coli or P. aeruginosa. This specific kind of killing occurs in vivo when drug concentrations are at least eight to 10 times higher than the MIC.

Verapamil analogues with restricted molecular flexibility: synthesis and pharmacological evaluation of the four isomers of alpha-[1-[3-[N-[1- [2-(3,4-dimethoxyphenyl)ethyl]]-N-methylamino]cyclohexyl]]-alpha- isopropyl-3,4-dimethoxybenzene-acetonitrile.

The synthesis and pharmacological activities of the four isomeric racemates of alpha-[1-[3-[N-[1-[2-(3,4-dimethoxyphenyl)ethyl]]-N- methylamino]cyclohexyl]]-alpha-isopropyl-3,4-dimethoxybenzene-acetoni trile are reported (2a-d). The compounds are verapamil analogues with restricted molecular flexibility designed to gather information on the active conformation(s) of the parent drug. The relative stereochemistry of the four racemates was established by X-ray crystallography and by 1H NMR spectroscopy; conformational analysis was supported by theoretical calculations. Negative inotropic and chronotropic activities were evaluated on guinea pig atria, while vasodilatory activity on smooth muscle was tested on guinea pig aortic strips. Binding studies on cat ventricles were performed using (-)-[N-methyl-3H]desmethoxyverapamil (D888) as a reference ligand. The results seem to support the hypothesis that cardiac depressant and vasorelaxant activities are due to different conformations of the verapamil molecule.

Clonal diversity of Shiga toxin-producing Escherichia coli O103:H2/H(-) in Germany.

Shiga toxin producing Escherichia coli O103:H2/H(-) belong to the third most frequently isolated EHEC serotypes in Germany following isolates of O157:H7/H(-) and O26:H11/H(-). A total of 145 respective E. coli 103 isolates from single cases of diarrhoea and haemolytic uremic syndrome (HUS) in 1997-2000 were characterised by a range of molecular subtyping methods (PFGE, P-gene profiling, ribotyping, electrotyping) and phage typing in order to analyse their genetic relatedness and the practicability for new epidemiological tracing back. All isolates cluster into a distinct EHEC subgroup and reveal a high clonal diversity together with a considerable stability. Since strains of this serotype rank up to the third most frequently isolated EHEC in Germany a large population of this serotype, and therefore, a great supply of such strains may exist in this country.

Malignant angioendotheliomatosis--a true lymphoma: a case of intravascular malignant lymphomatosis studied by southern blot hybridization analysis.

Malignant angioendotheliomatosis is a rare, systemic, usually fatal disease characterized by a massive proliferation of large, bizarre-looking mononuclear cells within small and medium-sized blood vessels. The histogenesis of the neoplastic cells has been the subject of long-standing controversy since the disease's initial description. Early investigators concluded that the entity represented a neoplasm of endothelial cells, but recently others have suggested that it is of lymphoid origin. We studied a case of malignant angioendotheliomatosis by Southern blot hybridization analysis which showed clonal rearrangements of the immunoglobulin heavy-chain gene strongly suggesting a B-lymphocyte origin. Our results provide additional evidence that malignant angioendotheliomatosis is an intravascular malignant lymphomatosis.

The role of insulin-like growth factor I and its receptor in cell growth, transformation, apoptosis, and chemoresistance in solid tumors.

Insulin-like growth factor I (IGF-I) exerts pleiotropic effects on mammalian cells via stimulation of its receptor (IGF-IR), a receptor tyrosine kinase. In vivo, IGF-I acts both as a local tissue growth factor and as a circulating hormone. In oncological research, IGF-I has received increased attention as the activated IGF-I/IGF-IR system displays mitogeneic, transforming, and anti-apoptotic properties in various cell types by stimulating distinct intracellular signaling pathways. Recent data suggest that the anti-apoptotic effect of IGF-I may mediate decreased sensitivity to chemotherapeutic drugs in vitro and in vivo. Thus, targeting the IGF-I/IGF-IR system could serve as an approach to overcome clinical drug resistance in certain tumors.

Sequence analysis of the DNA binding domain of the estrogen receptor gene in ER (+)/PR (-) breast cancer.

Estrogen stimulates the proliferation of breast cancer cells and regulates the expression of other proteins, including the progesterone receptor (PR), via interaction with a unique estrogen receptor (ER), a ligand-inducible transcription factor that binds to regulatory DNA sequences associated with target genes. The best indirect evidence of an intact ER gene signaling system in a tumor is the demonstration of both ER and PR cytosol protein. The molecular basis of the ER (+)/PR (-) phenotype is unknown and may reflect either defective PR gene expression or alterations in the ER-specifically, inability of the ligand-receptor complex to effectively bind to regulatory sequences in DNA. To test the latter possibility, we evaluated 10 ER (+)/PR (-) resected human breast cancers for small deletions and point mutations in the DNA binding domain of the ER gene. Exons 2 and 3 and their flanking intron sequences were selectively amplified using the polymerase chain reaction and then directly sequenced using the Sanger dideoxynucleotide method. A normal gene sequence was found in all cases studied. We conclude that sequence aberrations in the DNA binding domain of the ER are not a common cause of absent PR expression in ER (+)/PR (-) breast carcinomas.

Nourseothricin (streptothricin) inactivated by a plasmid pIE636 encoded acetyl transferase of Escherichia coli: location of the acetyl group.

Escherichia coli strains harbouring the plasmid pIE636 are able to synthesize acetylcoenzyme A: streptothricin acetyltransferase (ACSAT). The (enzymatic) N-acetylation of streptothricin F is known to contribute significantly towards the loss of antibacterial activity. 13C-NMR analysis of [14C]N-acetyl-labelled streptothricin F, produced by ACSAT-catalysed acetylation of streptothricin F and subsequent purification by various chromatographical steps, unequivocally revealed streptothricin F to be acetylated at the beta-amino group (C16) (and not at the epsilon-amino group (C19)).

Characterization, localization and pharmacological profile of a high-affinity [3H]lidocaine binding site.

Electrophysiological findings support the existence of voltage-dependent, sodium channel-associated receptors for class I antiarrhythmics. We have tried to identify such receptors with tritiated lidocaine. High-affinity binding sites were discovered in heart and brain membranes, but liver and kidney particulate fractions had the highest density of sites. The dissociation constants were 75 nM in bovine heart and 29 nM in guinea-pig liver membranes. Binding was reversible (t 1/2: 102 s at 2 degrees C), optimal at pH 9-10 and was only partly destroyed by heat treatment. Subcellular fractionation experiments excluded a plasmalemmal association of the lidocaine site in heart. The competition profile of 16 antiarrhythmics indicated chemical comparability of the sites in heart and liver. These data greatly challenge the applicability of labeled lidocaine as sodium channel probe. The pharmacological significance of the site described here remains to be clarified.

Structural dependence of the allosteric interaction of semi-rigid verapamil analogues with dihydropyridine-binding in kitten heart.

Structural determinants of the allosteric interaction of semi-rigid verapamil analogues with dihydropyridine binding were investigated in kitten heart using [3H](+)-isradipine as radioligand. Chemical variations were performed in the alkyl chain of verapamil and include introduction of unsaturation (double or triple bonds) or the insertion of cyclohexyl moieties. Introduction of unsaturation generally reduces the allosteric interaction in the case of 'double bond'-and abolishes it in the case of 'triple bond'-derivatives. Also the introduction of cyclohexyl moieties diminishes the potency of allosteric interaction: derivatives with the phenylethylamino side chain in an equatorial position exhibit the allosteric interaction, while it is lacking in derivatives with the basic side chain in axial position. Thus, the reduced conformational flexibility of the new verapamil congeners reduces or abolishes their ability to allosterically interfere with dihydropyridine binding. A molecular interpretation was approached by molecular modelling studies. The strategy was to find low energy conformations common to the active congeners, but not shared by the inactive ones. Structural features discriminating allosterically active and inactive congeners comprise: 1) the position of the nitrogen, 2) the volume occupied by the N-methyl groups, 3) the direction of the N-H bond and 4) the position of the phenyl ring in the basic side chain.