SAPROCAN: Saracatinib (AZD0530) and docetaxel in metastatic,castrate-refractory prostate cancer (mCRPC)—A phase I/randomized phase II study by the United Kingdom National Cancer Research Institute Prostate Group

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Background: Saracatinib is an orally-available, highly selective inhibitor of Abl and Src family members. It is an ATP-competitive tyrosine kinase inhibitor. Preclinical data suggested that the combination of a Src kinase inhibitor and docetaxel is synergistic, and Src kinase activity was also implicated in the bone’s metabolic response to cancer metastases. Methods: Patients with mCRPC were initially enrolled in an open-label, dose escalation phase I trial of oral saracatinib (cohorts of 50mg, 125mg and 175mg daily) with docetaxel (75mg/m2) in a 3+3 design. Subsequent patients were randomised 1:1 between saracatinib 175mg and placebo once daily. Pharmacokinetics (PK) of docetaxel were explored in phase I to exclude significant drug-drug interaction. The primary endpoint of phase II was biochemical or radiographic progression free survival (PFS). Secondary endpoints included overall survival (OS), safety and tolerability. Changes in circulating tumour cell (CTC) counts were also measured. The phase II was designed with a 1-sided alpha of 0.2 with 90% power to detect a hazard ratio (HR) for PFS of 0.67. Results: 10 patients were enrolled in phase I and 142 in the randomised phase II. No dose limiting toxicities or PK interactions were observed and the recommended dose for phase II was 175mg saracatinib daily and 75mg/m2docetaxel every 21 days. In phase II, the HR for PFS was 1.35 (80% confidence interval (CI) 1.07 to 1.70). The HR for OS was 1.42 (1.08 – 1.81). 41/71 and 29/71 experienced treatment related toxicities of grade 3 or above in the saracatinib and placebo arms respectively. 10/19 (53%) and 14/27 (52%) evaluable patients demonstrated a reduction in CTCs from ≥5 to < 5 /7.5ml blood at 6 weeks after starting saracatinib and placebo respectively. Conclusions: Saracatinib, in combination with docetaxel, adds toxicity and not efficacy in mCRPC. This combination should not be developed further in combination with docetaxel in the treatment of mCRPC. Clinical trial information: ISRCTN22566729.

Optimisation of an Electrochemical DNA Sensor for Measuring KRAS G12D and G13D Point Mutations in Different Tumour Types

Circulating tumour DNA (ctDNA) is widely used in liquid biopsies due to having a presence in the blood that is typically in proportion to the stage of the cancer and because it may present a quick and practical method of capturing tumour heterogeneity. This paper outlines a simple electrochemical technique adapted towards point-of-care cancer detection and treatment monitoring from biofluids using a label-free detection strategy. The mutations used for analysis were the KRAS G12D and G13D mutations, which are both important in the initiation, progression and drug resistance of many human cancers, leading to a high mortality rate. A low-cost DNA sensor was developed to specifically investigate these common circulating tumour markers. Initially, we report on some developments made in carbon surface pre-treatment and the electrochemical detection scheme which ensure the most sensitive measurement technique is employed. Following pre-treatment of the sensor to ensure homogeneity, DNA probes developed specifically for detection of the KRAS G12D and G13D mutations were immobilized onto low-cost screen printed carbon electrodes using diazonium chemistry and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride/N-hydroxysuccinimide coupling. Prior to electrochemical detection, the sensor was functionalised with target DNA amplified by standard and specialist PCR methodologies (6.3% increase). Assay development steps and DNA detection experiments were performed using standard voltammetry techniques. Sensitivity (as low as 0.58 ng/μL) and specificity (>300%) was achieved by detecting mutant KRAS G13D PCR amplicons against a background of wild-type KRAS DNA from the representative cancer sample and our findings give rise to the basis of a simple and very low-cost system for measuring ctDNA biomarkers in patient samples. The current time to receive results from the system was 3.5 h with appreciable scope for optimisation, thus far comparing favourably to the UK National Health Service biopsy service where patients can wait for weeks for biopsy results.

Developing a Low-Cost, Simple-to-Use Electrochemical Sensor for the Detection of Circulating Tumour DNA in Human Fluids

It is well-known that two major issues, preventing improved outcomes from cancer are late diagnosis and the evolution of drug resistance during chemotherapy, therefore technologies that address these issues can have a transformative effect on healthcare workflows. In this work we present a simple, low-cost DNA biosensor that was developed specifically to detect mutations in a key oncogene (KRAS). The sensor employed was a screen-printed array of carbon electrodes, used to perform parallel measurements of DNA hybridisation. A DNA amplification reaction was developed with primers for mutant and wild type KRAS sequences which amplified target sequences from representative clinical samples to detectable levels in as few as twenty cycles. High levels of sensitivity were demonstrated alongside a clear exemplar of assay specificity by showing the mutant KRAS sequence was detectable against a significant background of wild type DNA following amplification and hybridisation on the sensor surface. The time to result was found to be 3.5 h with considerable potential for optimisation through assay integration. This quick and versatile biosensor has the potential to be deployed in a low-cost, point-of-care test where patients can be screened either for early diagnosis purposes or monitoring of response to therapy.

A phase I study of olaparib in combination with capecitabine-based chemoradiation (CRT) in patients (pts) with locally advanced pancreatic cancer (LAPC)

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Background: Olaparib is a potent inhibitor of PARP-1, which has a critical role in signalling DNA single strand breaks (SSB) as part of the base excision repair pathway, and may have radio-sensitizing effects due to impaired resolution of radiation induced SSB. We hypothesize that O may potentiate the effects of X-CRT in pts with LAPC. Methods: Eligible pts with LAPC, ECOG < 1, tumor diameter < 6cm, with stable disease (SD) or response after 12 weeks’ induction chemotherapy, were treated with 1 of 4 escalating doses of O given bid po starting on day -3, and then in combination with X (830 mg/m2 bid) and radiation (50·4 Gy in 28 fractions) all administered Mon-Fri. Dose limiting toxicities (DLT) were determined on clinical and lab toxicity assessments (NCI-CTC AE v4.03) performed weekly from the start of O until completion of O plus X-CRT (i.e. 6 weeks). Dose escalation continued with a rolling-six design until the Maximum Tolerated Dose (MTD) was reached. Blood samples for PK analyses of O and PD measurement (inhibition of PARP activity) were collected on day -3 (O monotherapy) and during week 1 of O + X-CRT. Results: 18 pts, (9 m, 9 f, ECOG 0/1 [n=6/12]), age range 49-81 (median=70) years, with histologic (14) or cytologic (4) proven LAPC, had received induction chemotherapy with gemcitabine [GEM] (n=2), GEM + X (12), or FOLFIRINOX (3) with partial response (n=4) or stable disease (14). Pts received 50 (3), 100 (4), 150 (6), or 200 (5) mgs bid of O with X+CRT. DLTs were observed in 2 pts (both at 200mgs bid): 1 pt with grade 3 nausea (on optimal anti-emetics) and grade 3 fatigue, 1 pt with grade 3 anorexia. 6 pts were subsequently recruited at 150mgs bid with no DLTs. No pts had complete response, 2 pts had partial response (1 pt each at 100 and 150 mgs bid) and 1 pt (at 100 mgs bid) had progressive disease; the remaining 14 pts had SD. Conclusions: The recommended dose (RP2) of O is 150mgs bid when given in combination with X + CRT in LAPC. Recruitment of up to 12 pts with borderline operable LAPC at the RP2 is ongoing. PK analyses of O, PD studies (PARP inhibition – PBMCs; cytokeratin 18 – serum; γH2AX foci – hair follicles), and exploratory predictive marker studies (tumor – NGS; RNA exome sequencing) are ongoing. Clinical trial information: ISRCTN10361292.

A phase I trial of the oral hedgehog inhibitor taladegib (LY2940680) in combination with weekly paclitaxel in patients with advanced, solid tumours

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Background: Aberrant Hedgehog (Hh) signaling is implicated in carcinogenesis and is associated with poor prognosis in multiple tumours types. Hh inhibitors increase sensitivity to paclitaxel in taxane-resistant cell lines. Taladegib is an orally bioavailable, potent inhibitor of Smoothened, a key Hh pathway component, with activity in basal cell carcinoma. The single agent recommended dose is 400mg od. We present the dose escalation phase of a phase I study of weekly paclitaxel with oral taladegib. Methods: Primary objective: determine the dose limiting toxicity (DLT) and maximum tolerated dose (MTD) of taladegib on a continuous oral daily dosing regimen in combination with paclitaxel (80mg/m2, iv, day 1, 8 and 15 q 28) in patients with advanced solid cancers. Secondary objectives: assess the safety and tolerability, determine the recommended phase II dose (RP2D), and evaluate the pharmacokinetics of taladegib and paclitaxel. Exploratory objective: assess preliminary efficacy. A standard 3 + 3 dose escalation design was used. All patients received up to 6 cycles of paclitaxel. In addition, successive cohorts received continuous oral taladegib continued until progression or unacceptable toxicity as follows: dose level 1: 100mg od; 2: 200mg od; 3: 400mg od. Results: No DLTs were seen at dose level 1 or in the first 3 patients at dose level 2. 3 DLTs of grade 2 neuropathy were seen at dose level 3 (400mg taladegib); therefore, dose level 2 was expanded to 6 patients. No DLT was seen in the fourth patient and 2 additional patients have started treatment. After the DLT period 2 patients developed G2 and 4 developed G1 neuropathy. Other non DLT, drug-related G3 toxicities: uncomplicated neutropenia x2, muscle cramp x1 and fatigue x1. To date, 3 patients have had partial responses. Conclusions: The combination of daily oral taladegib and weekly paclitaxel is feasible. DLT of G2 neuropathy was seen at 400mg. Promising activity has been seen in solid tumours. A dose expansion cohort is due to commence in high grade ovarian carcinoma. ISRCTN No:ISRCTN15903698 Eudract Ref:2014-004695-37 Funded by Cancer Research UK C8361/A18775 and Ignyta. Sponsored by NHS Greater Glasgow and Clyde. Clinical trial information: ISRCTN15903698.

Primary resistance to integrase strand-transfer inhibitors in Europe

OBJECTIVES

The objective of this study was to define the natural genotypic variation of the HIV-1 integrase gene across Europe for epidemiological surveillance of integrase strand-transfer inhibitor (InSTI) resistance.

METHODS

This was a multicentre, cross-sectional study within the European SPREAD HIV resistance surveillance programme. A representative set of 300 samples was selected from 1950 naive HIV-positive subjects newly diagnosed in 2006-07. The prevalence of InSTI resistance was evaluated using quality-controlled baseline population sequencing of integrase. Signature raltegravir, elvitegravir and dolutegravir resistance mutations were defined according to the IAS-USA 2014 list. In addition, all integrase substitutions relative to HXB2 were identified, including those with a Stanford HIVdb score ≥ 10 to at least one InSTI. To rule out circulation of minority InSTI-resistant HIV, 65 samples were selected for 454 integrase sequencing.

RESULTS

For the population sequencing analysis, 278 samples were retrieved and successfully analysed. No signature resistance mutations to any of the InSTIs were detected. Eleven (4%) subjects had mutations at resistance-associated positions with an HIVdb score ≥ 10. Of the 56 samples successfully analysed with 454 sequencing, no InSTI signature mutations were detected, whereas integrase substitutions with an HIVdb score ≥ 10 were found in 8 (14.3%) individuals.

CONCLUSIONS

No signature InSTI-resistant variants were circulating in Europe before the introduction of InSTIs. However, polymorphisms contributing to InSTI resistance were not rare. As InSTI use becomes more widespread, continuous surveillance of primary InSTI resistance is warranted. These data will be key to modelling the kinetics of InSTI resistance transmission in Europe in the coming years.

Analysis of Human Dopamine D3 Receptor Quaternary Structure

The dopamine D3 receptor is a class A, rhodopsin-like G protein-coupled receptor that can form dimers and/or higher order oligomers. However, the molecular basis for production of these complexes is not well defined. Using combinations of molecular modeling, site-directed mutagenesis, and homogenous time-resolved FRET, the interfaces that allow dopamine D3 receptor monomers to interact were defined and used to describe likely quaternary arrangements of the receptor. These were then compared with published crystal structures of dimeric β1-adrenoreceptor, μ-opioid, and CXCR4 receptors. The data indicate important contributions of residues from within each of transmembrane domains I, II, IV, V, VI, and VII as well as the intracellular helix VIII in the formation of D3-D3 receptor interfaces within homo-oligomers and are consistent with the D3 receptor adopting a β1-adrenoreceptor-like quaternary arrangement. Specifically, results suggest that D3 protomers can interact with each other via at least two distinct interfaces: the first one comprising residues from transmembrane domains I and II along with those from helix VIII and a second one involving transmembrane domains IV and V. Moreover, rather than existing only as distinct dimeric species, the results are consistent with the D3 receptor also assuming a quaternary structure in which two transmembrane domain I-II-helix VIII dimers interact to form a "rhombic" tetramer via an interface involving residues from transmembrane domains VI and VII. In addition, the results also provide insights into the potential contribution of molecules of cholesterol to the overall organization and potential stability of the D3 receptor and possibly other GPCR quaternary structures.

Functional homomers and heteromers of dopamine D2L and D3 receptors co-exist at the cell surface

Human dopamine D(2long) and D(3) receptors were modified by N-terminal addition of SNAP or CLIP forms of O(6)-alkylguanine-DNA-alkyltransferase plus a peptide epitope tag. Cells able to express each of these four constructs only upon addition of an antibiotic were established and used to confirm regulated and inducible control of expression, the specificity of SNAP and CLIP tag covalent labeling reagents, and based on homogenous time-resolved fluorescence resonance energy transfer, the presence of cell surface D(2long) and D(3) receptor homomers. Following constitutive expression of reciprocal constructs, potentially capable of forming and reporting the presence of cell surface D(2long)-D(3) heteromers, individual clones were assessed for levels of expression of the constitutively expressed protomer. This was unaffected by induction of the partner protomer and the level of expression of the partner required to generate detectable cell surface D(2long)-D(3) heteromers was defined. Such homomers and heteromers were found to co-exist and using a reconstitution of function approach both homomers and heteromers of D(2long) and D(3) receptors were shown to be functional, potentially via trans-activation of associated G protein. These studies demonstrate the ability of dopamine D(2long) and D(3) receptors to form both homomers and heteromers, and show that in cells expressing each subtype a complex mixture of homomers and heteromers co-exists at steady state. These data are of potential importance both to disorders in which D(2long) and D(3) receptors are implicated, like schizophrenia and Parkinson disease, and also to drugs exerting their actions via these sites.

Evaluation of division-arrested cells for cell-based high-throughput screening and profiling

Just-in-time cell supply for cell-based high-throughput screening (HTS) is frequently problematic. In addition to scheduling and logistical issues, quality issues and variability due to passage effect, cell cycle, or confluency contribute to day-to-day signal variability in the course of cell-based HTS campaigns. Cell division-arrest and cryopreservation technologies permit the use of cells as assay-ready reagents for HTS and other cell-based profiling and structure-activity studies. In this report, the authors compare division-arrested and dividing cells in 2 assay types that are dependent on movement of proteins within or through cell membranes: a receptor tyrosine kinase assay involving A431 cells responsive to epidermal growth factor, and a secretion reporter assay, which measures secretion of a reporter gene, secreted alkaline phosphatase. In both assays, dividing and division-arrested cells yielded similar basal and maximal signals at a given cell density. Similar IC50s were obtained for reference inhibitors in each assay, type in both dividing and division-arrested cells. In addition, for the secretion reporter assay, when comparing IC50s obtained from 44 compounds randomly chosen from a primary screening hit list, the rank order of potency obtained from dividing cells and division-arrested cells was essentially identical. Furthermore, the results show that, under certain assay conditions, data generated using division-arrested cells are less variable than those generated using dividing cells. In summary, the results suggest that, in many cases, division-arrested cells can substitute for dividing cells and offer certain advantages for cell-based assays.

Effects of galanin receptor agonists on locus coeruleus neurons

Galanin exerts an inhibitory effect on locus coeruleus (LC) neurons via a postsynaptic, as yet unidentified galanin receptor. Using an in vitro intracellular recording technique the effect of two galanin receptor agonists on LC neurons was investigated. Bath application of [Sar(1), D-Ala(12)]gal(1-16)-NH(2) (AR-M961), an agonist both at galanin R1 and R2 (GALR1, GALR2) receptors, evoked a reversible membrane hyperpolarization and inhibition of spike discharge in all LC neurons tested (n=42). The action of AR-M961 was blocked by tetraethylammonium chloride. Hyperpolarizing responses induced by AR-M961 were retained in the presence of tetrodotoxin and high Mg(2+)/low Ca(2+) media. The selective GALR2 agonist Gal(2-11)-NH(2) (AR-M1896) only caused inhibition of spike discharge and a slight hyperpolarization in 26 of 34 LC neurons tested, and was on a molar basis much weaker than AR-M961. These results suggest that it mainly is the GALR1 receptor that mediates hyperpolarization of LC neurons.

Receptor subtype-specific pronociceptive and analgesic actions of galanin in the spinal cord: selective actions via GalR1 and GalR2 receptors

Galanin is a 29-aa neuropeptide with a complex role in pain processing. Several galanin receptor subtypes are present in dorsal root ganglia and spinal cord with a differential distribution. Here, we describe a generation of a specific galanin R2 (GalR2) agonist, AR-M1896, and its application in studies of a rat neuropathic pain model (Bennett). The results show that in normal rats mechanical and cold allodynia of the hindpaw are induced after intrathecal infusion of low-dose galanin (25 ng per 0.5 microl/h). The same effect is seen with equimolar doses of AR-M1896 or AR-M961, an agonist both at GalR1 and GalR2 receptors. In allodynic Bennett model rats, the mechanical threshold increased dose-dependently after intrathecal injection of a high dose of AR-M961, whereas no effect was observed in the control or AR-M1896 group. No effect of either of the two compounds was observed in nonallodynic Bennett model rats. These data indicate that a low dose of galanin has a nociceptive role at the spinal cord level mediated by GalR2 receptors, whereas the antiallodynic effect of high-dose galanin on neuropathic pain is mediated by the GalR1 receptors. Thus, a selective GalR1 agonist may be used to treat neuropathic pain.

The glycine residue in cyclic lactam analogues of galanin(1-16)-NH2 is important for stabilizing an N-terminal helix

The neuropeptide galanin is a 29- or 30-residue peptide whose physiological functions are mediated by G-protein-coupled receptors. Galanin's agonist activity has been shown to be associated with the N-terminal sequence, galanin(1-16). Conformational investigations previously carried out on full-length galanin have, furthermore, indicated the presence of a helical conformation in the neuropeptide's N-terminal domain. Several cyclic lactam analogues of galanin(1-16)-NH2 were prepared in an attempt to stabilize an N-terminal helix in the peptide. Here we describe and compare the solution conformational properties of these analogues in the presence of SDS micelles as determined by NMR, CD, and fluorescence spectroscopy. Differences in CD spectral profiles were observed among the compounds that were studied. Both c[D4, K8]Gal(1-16)-NH2 and c[D4,K8]Gal(1-12)-NH2 adopted stable helical conformations in the micelle solution. On the basis of the analyses of their respective alpha H chemical shifts and NOE patterns, this helix was localized to the first 10 residues. The distance between the aromatic rings of Trp2 and Tyr9 in c[D4, K8]Gal(1-16)-NH2 was determined to be 10.8 +/- 3 A from fluorescence resonance energy transfer measurements. This interchromophore spacing was found to be more consistent with a helical structure than an extended one. Removal of the Gly1 residue in compounds c[D4,K8]Gal(1-16)-NH2 and c[D4, K8]Gal(1-12)-NH2 resulted in a loss of helical conformation and a concomitant reduction in binding potency at the GalR1 receptor but not at the GalR2 receptor. The nuclear Overhauser enhancements obtained for the Gly1 deficient analogues did, however, reveal the presence of nascent helical structures within the N-terminal sequence. Decreasing the ring structure size in c[D4, K8]Gal(1-16)-NH2 by replacing Lys8 with an ornithine residue or by changing the position of the single lysine residue from eight to seven was accompanied by a complete loss of helical structure and dramatically reduced receptor affinity. It is concluded from the data obtained for the series of cyclic galanin(1-16)-NH2 analogues that both the ring structure size and the presence of an N-terminal glycine residue are important for stabilizing an N-terminal helix in these compounds. However, although an N-terminal helix constitutes a predominant portion of the conformational ensemble for compounds c[D4,K8]Gal(1-16)-NH2 and c[D4, K8]Gal(1-12)-NH2, these peptides nevertheless are able to adopt other conformations in solution. Consequently, the correlation between the ability of the cyclic galanin analogues to adopt an N-terminal helix and bind to the GalR1 receptor may be considered as a working hypothesis.

The receptor for the orexigenic peptide melanin-concentrating hormone is a G-protein-coupled receptor

Gene-knockout studies of melanin-concentrating hormone (MCH) and its effect on feeding and energy balance have firmly established MCH as an orexigenic (appetite-stimulating) peptide hormone. Here we identify MCH as the ligand for the orphan receptor SLC-1. The rat SLC-1 is activated by nanomolar concentrations of MCH and is coupled to the G protein G alpha i/o. The pattern of SLC-1 messenger RNA expression coincides with the distribution of MCH-containing nerve terminals and is consistent with the known central effects of MCH. Our identification of an MCH receptor could have implications for the development of new anti-obesity therapies.

Cloning and characterization of a cDNA encoding a novel subtype of rat thyrotropin-releasing hormone receptor

A cDNA encoding a thyrotropin-releasing hormone (TRH) receptor expressed in the pituitary was previously cloned (De La Pena, P., Delgado, L. M., Del Camino, D., and Barros, F. (1992) Biochem. J. 284, 891-899; De La Pena, P., Delgado, L. M., Del Camino, D., and Barros, F. (1992) J. Biol. Chem. 267, 25703-25708; Duthie, S. M., Taylor, P. L., Anderson, J., Cook, J., and Eidne, K. A. (1993) Mol. Cell Endocrinol. 95, R11-R15). We now describe the isolation of a rat cDNA encoding a novel subtype of TRH receptor (termed TRHR2) displaying an overall homology of 50% to the pituitary TRH receptor. Introduction of TRHR2 cDNA in HEK-293 cells resulted in expression of high affinity TRH binding with a different pharmacological profile than the pituitary TRH receptor. De novo expressed receptors were functional and resulted in stimulation of calcium transient as assessed by fluorometric imaging plate reader analysis. The message for TRHR2 was exclusive to central nervous system tissues as judged by Northern blot analysis. Studies of the expression of TRHR-2 message by in situ hybridization revealed a pattern of expression remarkably distinct (present in spinothalamic tract, spinal cord dorsal horn) from that of the pituitary TRH receptor (present in hypothalamus, and ventral horn of the spinal cord, anterior pituitary). Therefore, we have identified a novel, pharmacologically distinct receptor for thyrotropin-releasing hormone that appears to be more restricted to the central nervous system particularly to the sensory neurons of spinothalamic tract and spinal cord dorsal horn, which may account for the sensory antinociceptive actions of TRH.

The human 5-HT1A receptor: comparison of its binding properties in transfected cells and cortical tissue

1. The binding characteristics of tritium labeled 8-hydroxy-dipropyl-aminotetralin, or [3H]8-OH-DPAT, to the serotonin1A (5-HT1A) receptor in the stably transfected HeLa cell clone HA6 and in human cortical tissue were examined and compared. 2. A series of kinetic studies of [3H]8-OH-DPAT binding to the transfected HA6 cell line demonstrated two components in both the association and the dissociation reactions. 3. In saturation experiments, at least two affinity states were unequivocally detected in the HA6 cell line and the human cortical tissue. Using isotopic dilutions, the binding isotherms were best fitted to a two-site model, and similar affinity values were obtained in both systems (KH approximately 1.1 nM and KL approximately 12-223 nM). 4. Most of the drugs used in competitions inhibited [3H]8-OH-DPAT binding, following a two-site model, and maintained their rank order of binding potency in both systems; that is, 5-HT > or = 8-OH-DPAT > buspirone > pindolol. Inconsistencies, however, were found for the antagonists NAN-190 and pindolol; only one inhibition constant was determined for HA6 cells, but two affinities were detected with cortical tissue. 5. The results indicate that, although data from binding studies using the cell expression system reflect, to a certain extent, those obtained with the cortical tissue, some discrepancies remained. 6. Finally, and in contrast with what is observed with the 5-HT1A receptor expressed in the HA6 cell line, it is possible that different receptors, or subtypes of one receptor, or even uptake sites normally expressed in cortical tissue, could interact with [3H]8-OH-DPAT or the competing drugs or both, thus leading to the observation of additional binding sites.

The human 5-hydroxytryptamine 1A receptor differentially modulates phospholipase C and adenylyl cyclase activities

In this study, we quantitate and compare the ability of the 5-hydroxytryptamine 1A (5-HT1A) receptor to modulate the activities of phospholipase C and adenylyl cyclase as a function of receptor concentration. We used a single clonal cell line permanently expressing the human 5-HT1A receptor, and progressively depleted the receptor concentration using an alkylating antagonist (N8-bromoacetyl-N1-3'-(4-indolyloxy)-2'-hydroxypropyl-Z-1,8-diamin o-p-methane, (+-) Pindobind). For serotonin-induced phospholipase C stimulation, reductions in receptor number result in dose-response curves that shift downward and rightward, reflecting both a decreasing maximal effect as well as an increasing ED50. In contrast, depletion of more than 95% of the receptors has no effect on the maximal inhibition of forskolin-stimulated adenylyl cyclase activity. Moreover, at all receptor concentrations, the amount of serotonin required to produce half-maximal phospholipase C stimulation is several-fold more than that required to produce half-maximal inhibition of adenylyl cyclase activity. We conclude that the 5-HT1A receptor modulates these two pathways differently, and that the overall response to challenge with serotonin, in terms of both phosphatidyl inositol hydrolysis and cyclic AMP production, is dependent upon receptor number.