Sialic Acid Mediated Endothelial and Hepatic Uptake: A Mechanism based Mathematic Model Elucidating the Complex Pharmacokinetics and Pharmacodynamics of Efmarodocokin Alfa, a Variably Glycosylated Fusion Protein

Sialic acid (SA) is crucial for protecting glycoproteins from clearance. Efmarodocokin alfa (IL-22Fc), a fusion protein agonist that links IL-22 to the crystallizable fragment (Fc) of human IgG4, contains 8 N-glycosylation sites and exhibits heterogeneous and variable terminal sialylation biodistribution. This presents a unique challenge for Pharmacokinetic (PK) and Pharmacodynamic (PD) analysis and cross-species translation. In this study, we sought to understand how varying SA levels and heterogeneous distribution contribute to IL-22Fc's complex PKPD properties. We initially used homogenous drug material with varying SA levels to examine PKPD in mice. Population PKPD analysis based on mouse data revealed that SA was a critical covariate simultaneously accounting for the substantial between subject variability (BSV) in clearance (CL), distribution clearance (CLd), and volume of distribution (Vd). In addition to the well-established mechanism by which SA inhibits ASGPR activity, we hypothesized a novel mechanism by which decrease in SA increases the drug uptake by endothelial cells. This decrease in SA, leading to more endothelial uptake, was supported by the neonatal Fc receptor (FcRn) dependent cell-based transcytosis assay. The population analysis also suggested in vivo EC50 (IL-22Fc stimulating Reg3β) was independent on SA, while the in-vitro assay indicated a contradictory finding of SA-in vitro potency relationship. We created a mechanism based mathematical (MBM) PKPD model incorporating the decrease in SA mediated endothelial and hepatic uptake, and successfully characterized the SA influence on IL-22Fc PK, as well as the increased PK exposure being responsible for increased PD. Thereby, the MBM model supported that SA has no direct impact on EC50, aligning with the population PKPD analysis. Subsequently, using the MBM PKPD model, we employed 5 subpopulation simulations to reconstitute the heterogeneity of drug material. The simulation accurately predicted the PKPD of heterogeneously and variably sialylated drug in mouse, monkey and human. The successful prospective validation confirmed the MBM's ability to predict IL-22Fc PK across variable SA levels, homogenous to heterogeneous material, and across species (R2=0.964 for clearance prediction). Our model prediction suggests an average of 1 mol/mol SA increase leads to a 50% increase in drug exposure. This underlines the significance of controlling sialic acid levels during lot-to-lot manufacturing.

Mini-PBPK-Based Population Model and Covariate Analysis to Assess the Complex Pharmacokinetics and Pharmacodynamics of RO7449135, an Anti-KLK5/KLK7 Bispecific Antibody in Cynomolgus Monkeys

RO7449135, an anti-kallikrein (KLK)5/KLK7 bispecific antibody, is in development as a potential therapy against Netherton's syndrome (NS). In cynomolgus monkey studies, RO7449135 bound to KLK5 and KLK7, causing considerable accumulation of total KLKs, but with non-dose-proportional increase. To understand the complex PKPD, a population model with covariate analysis was developed accounting for target binding in skin and migration of bound targets from skin to blood. The covariate analysis suggested the animal batch as the categorical covariate impacting the different KLK5 synthesis rates between the repeat-dose study and single-dose study, and the dose as continuous covariate impacting the internalization rate of the binary and ternary complexes containing KLK7. To comprehend the mechanism underlying, we hypothesized that inhibition of KLK5 by RO7449135 prevented its cleavage of the pro-enzyme of KLK7 (pro-KLK7) and altered the proportion between pro-KLK7 and KLK7. Besides the pro-KLK7, RO7449135 can interact with other proteins like LEKTI through KLK7 connection in a dose-dependent manner. The different high-order complexes formed by RO7449135 interacting with pro-KLK7 or LEKTI-like proteins can be subject to faster internalization rate. Accounting for the dose and animal batch as covariates, the model-predicted free target suppression is well aligned with the visual target engagement check. The population PKPD model with covariate analysis provides the scientific input for the complex PKPD analysis, successfully predicts the target suppression in cynomolgus monkeys, and thereby can be used for the human dose projection of RO7449135.

First-In-Human Phase I Study of the OX40 Agonist MOXR0916 in Patients with Advanced Solid Tumors

PURPOSE

OX40, a receptor transiently expressed by T cells upon antigen recognition, is associated with costimulation of effector T cells and impairment of regulatory T-cell function. This first-in-human study evaluated MOXR0916, a humanized effector-competent agonist IgG1 monoclonal anti-OX40 antibody.

PATIENTS AND METHODS

Eligible patients with locally advanced or metastatic refractory solid tumors were treated with MOXR0916 intravenously once every 3 weeks (Q3W). A 3+3 dose-escalation stage (0.2-1,200 mg; n = 34) was followed by expansion cohorts at 300 mg (n = 138) for patients with melanoma, renal cell carcinoma, non-small cell lung carcinoma, urothelial carcinoma, and triple-negative breast cancer.

RESULTS

MOXR0916 was well tolerated with no dose-limiting toxicities observed. An MTD was not reached. Most patients (95%) experienced at least one adverse event (AE); 56% of AEs, mostly grade 1-2, were related to MOXR0916. Most common treatment-related AEs included fatigue (17%), diarrhea (8%), myalgia (7%), nausea (6%), decreased appetite (6%), and infusion-related reaction (5%). Pharmacokinetic (PK) parameters were dose proportional between 80 and 1,200 mg and supported Q3W administration. The recommended expansion dose based on PK and OX40 receptor saturation was 300 mg Q3W. Immune activation and upregulation of PD-L1 was observed in a subset of paired tumor biopsies. One renal cell carcinoma patient experienced a confirmed partial response. Overall, 33% of patients achieved stable disease.

CONCLUSIONS

Although objective responses were rarely observed with MOXR0916 monotherapy, the favorable safety profile and evidence of tumor immune activation in a subset of patients support further investigation in combination with complementary agents such as PD-1/PD-L1 antagonists.

An integrated approach for characterizing immunogenic responses toward a bispecific antibody

Bispecific antibodies (bsAbs) recognize and bind two different targets or two epitopes of the same antigen, making them an attractive diagnostic and treatment modality. Compared to the production of conventional bivalent monospecific antibodies, bsAbs require greater engineering and manufacturing. Therefore, bsAbs are more likely to differ from endogenous immunoglobulins and contain new epitopes that can increase immunogenic risk. Anti-A/B is a bsAb designed using a ‘knobs-into-holes’ (KIH) format. Anti-A/B exhibited an unexpectedly high immunogenicity in both preclinical and clinical studies, resulting in early termination of clinical development. Here, we used an integrated approach that combined in silico analysis, in vitro assays, and an in vivo study in non-human primates to characterize anti-A/B immunogenicity. Our findings indicated that the immunogenicity is associated with epitopes in the anti-B arm and not with mutations engineered through the KIH process. Our results showed the value of this integrated approach for performing immunogenicity risk assessment during clinical candidate selection to effectively mitigate risks during bsAb development.

Randomized Phase I Healthy Volunteer Study of UTTR1147A (IL-22Fc): A Potential Therapy for Epithelial Injury

Most treatments for epithelial injury target hematopoietic mechanisms, possibly causing immunosuppression. Interleukin (IL)-22 promotes tissue regeneration, acting directly on epithelial cells. UTTR1147A, a human IL-22Fc (immunoglobulin G (IgG)4) fusion protein, activates IL-22 signaling. This phase I placebo-controlled trial of single, ascending, i.v. (1-120 μg/kg) and s.c (3-120 μg/kg) doses of UTTR1147A analyzed its effects on safety, tolerability, pharmacokinetics, and pharmacodynamic biomarkers in healthy volunteers. Most adverse events (AEs) were mild or moderate. The maximum tolerated i.v. dose in healthy volunteers was 90 μg/kg. Predominant AEs were dose-dependent reversible skin effects consistent with IL-22 pharmacology. UTTR1147A exposure increased approximately dose-proportionally, with a half-life of ~1 week. IL-22 biomarkers (regenerating islet protein 3A (REG3A), serum amyloid A (SAA), and C-reactive protein (CRP)) increased dose-dependently. Neither inflammatory symptoms and signs nor cytokines increased with CRP elevations. UTTR1147A demonstrated acceptable safety, pharmacokinetics, and IL-22R engagement, supporting further clinical development.

Preclinical pharmacokinetic characterization of an adipose tissue-targeting monoclonal antibody in obese and non-obese animals

Target receptor levels can influence pharmacokinetics (PK) or pharmacodynamics (PD) of monoclonal antibodies (mAbs), and can affect drug development of this class of molecules. We generated an effector-less humanized bispecific antibody that selectively activates fibroblast growth factor receptor (FGFR)1 and βKlotho receptor, a FGF21 receptor complex highly expressed in both white and brown adipocytes. The molecule shows cross-species binding with comparable equilibrium binding affinity (Kd) for human, cynomolgus monkey, and mouse FGFR1/βKlotho. To understand the PK/PD relationship in non-obese and obese animals, we evaluated the adipose tissue distribution of the antibody, serum exposures, and an associated PD marker (high-molecular-weight adiponectin), in both non-obese and obese mice and monkeys. Antibody uptake into fat tissue was found to be higher on a per gram basis in non-obese animals compared to obese animals. Since obesity has been reported to be associated with reduced expression of FGFR1 and βKlotho receptor in white adipose tissues in mice, our results suggest that the distribution in adipose tissues was influenced by target expression levels. Even so, the overall dose-normalized serum exposures were comparable between non-obese and obese mice and monkeys, suggesting that adipose tissue uptake plays a limited role in overall systemic PK determination. It remains to be determined if and how obesity and receptor expression in humans influence the PK and PD profile of this novel therapeutic candidate.

Abstract 3628: T cell-dependent bispecific antibody anti-CD79b/CD3 as a potential therapy for B-cell malignancies

T-cell recruiting bispecific antibodies and antibody fragments have been used to harness the cytotoxic potential of T cells for cancer treatment. We have adopted a knobs-into-holes antibody format and produced T-cell dependent bispecific antibodies (TDB), as full length, humanized IgG1 antibodies with natural antibody architecture, which allow one arm to target various B cell antigens while the other arm recruits T cells by binding to the CD3ϵ subunit of the T-cell receptor. One B cell antigen targeted is CD79b, a component of the B cell receptor complex, which has been clinically validated by an anti-CD79b antibody-drug conjugate (ADC) as a safe and effective therapeutic target for B cell malignancies (Leukemia 2015 Pfeifer et.al).

In the present work, we show that anti-CD79b/CD3 TDB is active against lymphoma cells with a wide range of CD79b antigen levels in vitro. In addition, anti-CD79b/CD3 TDB appears to be insensitive to the status of cellular signaling pathways in lymphoma cells as it is active against cell lines that are resistant to the BTK inhibitor ibrutinib or an anti-CD79b ADC with a cleavable tubulin inhibitor as the payload. In vivo, anti-CD79b/CD3 TDB administration inhibited tumor growth in B-cell lymphoma xenograft models and resulted in potent B-cell depletion in the blood and spleens in a humanized murine model.

To assess the safety of targeting CD79b with a T-cell recruiting bispecific antibody in non-human primates, a surrogate anti-cynoCD79b/CD3 TDB with comparable in vitro potency was produced with a target arm that recognizes cynomolgus monkey CD79b and the same anti-CD3 arm. In a single dose pharmacokinetic/pharmacodynamics/safety study, anti-cynoCD79b/CD3 TDB administration at 1mg/kg resulted in potent B cell depletion, as well as T cell activation and proliferation, which was assessed by flow cytometry on blood and lymphoid tissue and immunohistochemistry on lymphoid tissue. The pharmacokinetic properties of anti-cynoCD79b/CD3 TDB resembled that of IgG antibodies; though with a faster clearance likely due to CD79b antigen internalization and enhanced binding to CD3 when compared to previously described anti-CD20/CD3 TDB. Transient cytokine release was observed as elevated levels of IL-2, IL-6, IFN-gamma and TNF-alpha were detected within 24 hours following administration. Anti-cynoCD79b/CD3 TDB was well-tolerated in the majority of dosed animals without toxicologically significant findings.

Collectively, these preclinical data suggest anti-CD79b/CD3 TDB can be a potential therapeutic agent in B cell malignancies.

Citation Format: Peiyin Wang, Maria Hristopoulos, Robyn Clark, Yvonne Chen, Diego Ellerman, Mary Mathieu, Christoph Spiess, Jessica Li, Cecile Chalouni, Siddharth Sukumaran, Eric Stefanich, Jeffrey Wallin, Robert Li, Tanja Zabka, Klara Totpal, Mark Dennis, Allen Ebens, Stephen Gould, Andrew Polson, Liping Laura Sun. T cell-dependent bispecific antibody anti-CD79b/CD3 as a potential therapy for B-cell malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3628. doi:10.1158/1538-7445.AM2017-3628

Abstract B043: Systems pharmacology modeling of anti-CD20/CD3 T-cell dependent bispecific antibody and its application to clinical trial design

Objectives: To develop and apply a quantitative systems pharmacology (QSP) model to support clinical trial design for anti-CD20/CD3 T-cell dependent bispecific antibody in non-Hodgkin lymphoma (NHL).

Methods: We developed a mechanistic model to describe the dynamics of B- and T-lymphocytes and their interactions in multiple physiological compartments (peripheral blood, tumor, and lymphoid tissues including the spleen, lymph nodes, and bone marrow) in the presence of an anti-CD20/CD3 T-cell dependent bispecific antibody (CD20-TDB) and the CD19-targeting bispecifc T-cell engager blinatumomab (a molecule with a similar mechanism of action, approved for ALL). The model is based on physiological, mechanistic, pharmacokinetic, and pharmacodynamic data and includes: 1) multiple activation states of CD8+ T-cells; 2) CD19+CD20- (pro-B), and CD19+CD20+ (pre- to mature-B) B-lymphocytes; and 3) the pharmacokinetics of CD20-TDB and blinatumomab and their mechanistic effects (activation of CD8+ T-lymphocytes and consequent killing of CD20+ and CD19+ B-lymphocytes, respectively).

Results: The model was calibrated using in vitro potency data and circulating cell measurements in cynomolgous monkeys treated with CD20-TDB [1]. Preclinical and translational validation was performed using additional CD20-TDB preclinical data and blinatumomab clinical data [2, 3]. The model replicated the effects of CD20-TDB on cell dynamics in circulation, and predicted CD20-TDB preclinical data at lower doses and in tissues, and blinatumomab clinical data in patients with ALL and NHL. The model will be used to evaluate and propose dosing strategies for CD20-TDB in patients with NHL and to inform other clinical development considerations.

Conclusions: The mechanism-based systems model accurately captures and predicts the pharmacodynamics of CD20-TDB in cynomolgous monkeys and in patients with ALL and NHL. The model provides a novel approach for evaluation of clinical dosing strategies for CD20-TDB and can be extended to related molecules and/or other B cell malignancies.

References: [1] Sun LL, et al. Sci Transl Med. 2015; 7(287): 1-10. [2] Klinger M, et al. Blood. 2012;119(26):6226-3. [3] Hijazi Y, et al. Abstract: 3051. ASCO; May 31-June 4, 2013.

Citation Format: Iraj Hosseini, Kapil Gadkar, Eric Stefanich, Chi-Chung Li, Laura Sun, Yu-Waye Chu, Saroja Ramanujan. Systems pharmacology modeling of anti-CD20/CD3 T-cell dependent bispecific antibody and its application to clinical trial design [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B043.

Anti-CD20/CD3 T cell-dependent bispecific antibody for the treatment of B cell malignancies

Bispecific antibodies and antibody fragments in various formats have been explored as a means to recruit cytolytic T cells to kill tumor cells. Encouraging clinical data have been reported with molecules such as the anti-CD19/CD3 bispecific T cell engager (BiTE) blinatumomab. However, the clinical use of many reported T cell-recruiting bispecific modalities is limited by liabilities including unfavorable pharmacokinetics, potential immunogenicity, and manufacturing challenges. We describe a B cell-targeting anti-CD20/CD3 T cell-dependent bispecific antibody (CD20-TDB), which is a full-length, humanized immunoglobulin G1 molecule with near-native antibody architecture constructed using "knobs-into-holes" technology. CD20-TDB is highly active in killing CD20-expressing B cells, including primary patient leukemia and lymphoma cells both in vitro and in vivo. In cynomolgus monkeys, CD20-TDB potently depletes B cells in peripheral blood and lymphoid tissues at a single dose of 1 mg/kg while demonstrating pharmacokinetic properties similar to those of conventional monoclonal antibodies. CD20-TDB also exhibits activity in vitro and in vivo in the presence of competing CD20-targeting antibodies. These data provide rationale for the clinical testing of CD20-TDB for the treatment of CD20-expressing B cell malignancies.

Abstract S1-07: HER2 T cell dependent bispecific antibody (HER2-TDB) for treatment of HER2 positive breast cancer

Based on recent clinical success of tumor immunotherapies that block immune suppressive mechanisms to restore T cell function, there is a profound interest in the clinical development of T cell targeted therapies. We have produced a trastuzumab-based HER2 T cell dependent bispecific antibody (HER2-TDB) that conditionally activates T cells resulting in lysis of HER2 expressing cancer cells at low picomolar concentrations. Due to its unique mechanism of action, which is unrelated to HER2 signaling or sensitivity to chemotherapeutic agents, HER2-TDB can eliminate cells refractory to currently approved HER2 therapies. The potent anti-tumor activity of HER2-TDB was demonstrated using four model systems including MMTV-huHER2 and huCD3 transgenic mice. We demonstrate inhibitory effect of PD-L1 expression on the activity of bispecific T cell recruiting antibodies. This resistance mechanism is reversed by anti-PD-L1 treatment and combination of HER2-TDB with anti-PD-L1 immune therapy resulted in enhanced inhibition of tumor growth, increased response rates and durable responses.

Significance:

This report presents a new immunotherapy for HER2+ breast cancer with an alternative, extremely potent mechanism of action that is effective in cells resistant to current HER2 targeted therapies. Several significant advances are provided to bispecific T cell recruiting antibodies: we characterize a critical resistance mechanism, a potential diagnostic marker, a novel transgenic efficacy model and significantly improve the drug-like properties by using technology based on full length antibodies with natural architecture. Finally we demonstrate the benefit of combining two immune therapies: direct polyclonal recruitment of T cell activity together with inhibiting the T cell suppressive PD-1/PD-L1 signaling results in enhanced and durable long term responses.

Citation Format: Teemu T Junttila, Ji Li, Jennifer Johnston, Maria Hristopoulos, Robyn Clark, Diego Ellerman, Bu-Er Wang, Yijin Li, Mary Mathieu, Guangmin Li, Judy Young, Elizabeth Luis, Gail Lewis Phillips, Eric Stefanich, Cristoph Spiess, Andrew Polson, Bryan Irving, Justin M Scheer, Melissa R Junttila, Mark S Dennis, Robert Kelley, Klara Totpal, Allen Ebens. HER2 T cell dependent bispecific antibody (HER2-TDB) for treatment of HER2 positive breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr S1-07.

Functional consequences of the macrophage stimulating protein 689C inflammatory bowel disease risk allele

Background

Macrophage stimulating protein (MSP) is a serum growth factor that binds to and activates the receptor tyrosine kinase, Recepteur d'Origine Nantais (RON). A non-synonymous coding variant in MSP (689C) has been associated with genetic susceptibility to both Crohn's disease and ulcerative colitis, two major types of inflammatory bowel disease (IBD) characterized by chronic inflammation of the digestive tract. We investigated the consequences of this polymorphism for MSP-RON pathway activity and IBD pathogenesis.

Methods

RON expression patterns were examined on mouse and human cells and tissues under normal and disease conditions to identify cell types regulated by MSP-RON. Recombinant MSP variants were tested for their ability to bind and stimulate RON and undergo proteolytic activation. MSP concentrations were quantified in the serum of individuals carrying the MSP 689R and 689C alleles.

Results

In intestinal tissue, RON was primarily expressed by epithelial cells under normal and disease conditions. The 689C polymorphism had no impact on the ability of MSP to bind to or signal through RON. In a cohort of normal individuals and IBD patients, carriers of the 689C polymorphism had lower concentrations of MSP in their serum.

Conclusions

By reducing the quantities of circulating MSP, the 689C polymorphism, or a variant in linkage disequilibrium with this polymorphism, may impact RON ligand availability and thus receptor activity. Given the known functions of RON in regulating wound healing and our analysis of RON expression patterns in human intestinal tissue, these data suggest that decreased RON activity may impact the efficiency of epithelial repair and thus underlie the increased IBD susceptibility associated with the MSP 689C allele.

Development of a human IgG4 bispecific antibody for dual targeting of interleukin-4 (IL-4) and interleukin-13 (IL-13) cytokines

Human bispecific antibodies have great potential for the treatment of human diseases. Although human IgG1 bispecific antibodies have been generated, few attempts have been reported in the scientific literature that extend bispecific antibodies to other human antibody isotypes. In this paper, we report our work expanding the knobs-into-holes bispecific antibody technology to the human IgG4 isotype. We apply this approach to generate a bispecific antibody that targets IL-4 and IL-13, two cytokines that play roles in type 2 inflammation. We show that IgG4 bispecific antibodies can be generated in large quantities with equivalent efficiency and quality and have comparable pharmacokinetic properties and lung partitioning, compared with the IgG1 isotype. This work broadens the range of published therapeutic bispecific antibodies with natural surface architecture and provides additional options for the generation of bispecific antibodies with differing effector functions through the use of different antibody isotypes.

Production, characterization, and pharmacokinetic properties of antibodies with N-linked mannose-5 glycans

The effector functions of therapeutic antibodies are strongly affected by the specific glycans added to the Fc domain during post-translational processing. Antibodies bearing high levels of N-linked mannose-5 glycan (Man5) have been reported to exhibit enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) compared with antibodies with fucosylated complex or hybrid glycans. To better understand the relationship between antibodies with high levels of Man5 and their biological activity in vivo, we developed an approach to generate substantially homogeneous antibodies bearing the Man5 glycoform. A mannosidase inhibitor, kifunensine, was first incorporated in the cell culture process to generate antibodies with a distribution of high mannose glycoforms. Antibodies were then purified and treated with a mannosidase for trimming to Man5 in vitro. This 2-step approach can consistently generate antibodies with > 99% Man5 glycan. Antibodies bearing varying levels of Man5 were studied to compare ADCC and Fcγ receptor binding, and they showed enhanced ADCC activity and increased binding affinity to the FcγRIIIA. In addition, the clearance rate of antibodies bearing Man8/9 and Man5 glycans was determined in a pharmacokinetics study in mice. When compared with historical data, the antibodies bearing the high mannose glycoform exhibited faster clearance rate compared with antibodies bearing the fucosylated complex glycoform, while the pharmacokinetic properties of antibodies with Man8/9 and Man5 glycoforms appeared similar. In addition, we identified the presence of a mannosidase in mouse serum that converted most Man8/9 to Man6 after 24 h.

Equilibrative nucleoside transporter 3 deficiency perturbs lysosome function and macrophage homeostasis

Lysosomal storage diseases (LSDs) are a group of heterogeneous disorders caused by defects in lysosomal enzymes or transporters, resulting in accumulation of undegraded macromolecules or metabolites. Macrophage numbers are expanded in several LSDs, leading to histiocytosis of unknown pathophysiology. Here, we found that mice lacking the equilibrative nucleoside transporter 3 (ENT3) developed a spontaneous and progressive macrophage-dominated histiocytosis. In the absence of ENT3, defective apoptotic cell clearance led to lysosomal nucleoside buildup, elevated intralysosomal pH, and altered macrophage function. The macrophage accumulation was partly due to increased macrophage colony-stimulating factor and receptor expression and signaling secondary to the lysosomal defects. These studies suggest a cellular and molecular basis for the development of histiocytosis in several human syndromes associated with ENT3 mutations and potentially other LSDs.

MTRX1011A, a humanized anti-CD4 monoclonal antibody, in the treatment of patients with rheumatoid arthritis: a phase I randomized, double-blind, placebo-controlled study incorporating pharmacodynamic biomarker assessments

Introduction

The purpose of this study was to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of the humanized anti-CD4 monoclonal antibody MTRX1011A in a randomized, double-blind placebo-controlled Phase 1 study in patients with rheumatoid arthritis (RA).

Methods

In the single ascending dose (SAD) portion of the study, patients received single doses of a placebo or MTRX1011A at 0.3, 1.0, 3.5 and 7.0 mg/kg intravenously (IV) or 1.0 and 3.5 mg/kg subcutaneously (SC), followed by five weeks of evaluation. In the multi-dose (MD) portion of the study, placebo or MTRX1011A was administered weekly for eight doses at 1.5 or 3.5 mg/kg SC, or 5 mg/kg IV, followed by eight weeks of evaluation.

Results

MTRX1011A was well tolerated in the SAD phase up to 7 mg/kg IV and in the MD phase up to 1.5 mg/kg SC. At weekly doses of 3.5 mg/kg SC and 5 mg/kg IV, a moderate pruritic papular rash was observed in some MTRX1011A-treated patients, which was considered a dose-limiting toxicity for this clinical indication. No serious adverse events occurred in any cohort. Reduction in disease activity was modest. PD assessments demonstrated that MTRX1011A induced a dose-dependent down-modulation of CD4 expression on peripheral blood CD4 T cells, CD4 receptor occupancy, increases in serum sCD4-MTRX1011A complexes and up-regulation of CD69 on T cells, but was non-depleting.

Conclusions

The maximum tolerated dose of MTRX1011A was 1.5 mg/kg SC administered weekly. At this dose MTRX1011A did not achieve maximum PD activity expected to be required for reduction in disease activity.

Pharmacokinetics/pharmacodynamics of nondepleting anti-CD4 monoclonal antibody (TRX1) in healthy human volunteers

PURPOSE

TRX1 is a nondepleting anti-CD4 monoclonal IgG1 antibody being developed to induce tolerance by blocking CD4-mediated functions. The purpose of this study is to describe the pharmacokinetics (PK) and pharmacodynamics (PD) of TRX1 and to develop a receptor-mediated PK/PD model that characterizes the relationships between serum TRX1 concentration and total and free CD4 expression in healthy male volunteers.

METHODS

Nine subjects from three dosing cohorts in double-blinded, placebo-controlled phase I clinical study was included in the analysis. Serum TRX1 levels were determined using enzyme-linked immunosorbent assay. Blood total and free CD4 receptor levels were determined by using flow cytometric analyses. The receptor-mediated PK/PD model was developed to describe the dynamic interaction of TRX1 binding with CD4 receptors.

RESULTS AND CONCLUSIONS

TRX1 displayed nonlinear pharmacokinetic behavior and the CD4 receptors on T cells were saturated and down-modulated following treatment with TRX1. Results from in vitro studies using purified human T cells suggested that CD4-mediated internalization may constitute one pathway by which CD4 is down-modulated and TRX1 is cleared in vivo. The developed receptor-mediated PK/PD model adequately described the data. This PK/PD model was used to simulate PK/PD time profiles after different dosing regimens to help guide the dose selection in future clinical studies.

In vitro internalization, intracellular transport, and clearance of an anti-CD11a antibody (Raptiva) by human T-cells

Efalizumab (Raptiva) is a humanized CD11a-specific monoclonal antibody that was recently approved for the treatment of moderate to severe psoriasis. In psoriasis patients, the rate of efalizumab clearance from serum is related to T-cell surface expression of CD11a, suggesting a receptor-mediated clearance model for efalizumab (Bauer et al., 1999). However, limited experimental data are available to explain how the interaction with CD11a results in the systemic clearance of efalizumab. The following studies were designed to test the hypothesis that one mechanism of anti-CD11a antibody clearance is mediated in part by cellular internalization. This was tested in vitro using purified mouse and human T-cells as a model to study the cellular uptake and clearance of anti-CD11a antibodies. Data from these studies suggest that anti-CD11a antibodies are internalized by purified T-cells. Upon internalization, the antibodies appeared to be targeted to lysosomes and were cleared from within the cells in a time-dependent manner. CD11a-mediated internalization and lysosomal targeting of efalizumab may constitute one pathway by which this antibody is cleared in vivo.

Role of the distal half of the c-Mpl intracellular domain in control of platelet production by thrombopoietin in vivo

The cytokine thrombopoietin (TPO) controls the formation of megakaryocytes and platelets from hematopoietic stem cells. TPO exerts its effect through activation of the c-Mpl receptor and of multiple downstream signal transduction pathways. While the membrane-proximal half of the cytoplasmic domain appears to be required for the activation of signaling molecules that drive proliferation, the distal half and activation of the mitogen-activated protein kinase pathway have been implicated in mediating megakaryocyte maturation in vitro. To investigate the contribution of these two regions of c-Mpl and the signaling pathways they direct in mediating the function of TPO in vivo, we used a knock-in (KI) approach to delete the carboxy-terminal 60 amino acids of the c-Mpl receptor intracellular domain. Mice lacking the C-terminal 60 amino acids of c-Mpl (Delta60 mice) have normal platelet and megakaryocyte counts compared to wild-type mice. Furthermore, platelets in the KI mice are functionally normal, indicating that activation of signaling pathways connected to the C-terminal half of the receptor is not required for megakaryocyte differentiation or platelet production. However, Delta60 mice have an impaired response to exogenous TPO stimulation and display slower recovery from myelosuppressive treatment, suggesting that combinatorial signaling by both ends of the receptor intracellular domain is necessary for an appropriate acute response to TPO.

Metabolism of thrombopoietin (TPO) in vivo: determination of the binding dynamics for TPO in mice

Previous in vivo studies have established that plasma thrombopoietin (TPO) levels are regulated by binding to c-Mpl on platelets and that, in vitro, platelets bind and degrade TPO. To determine if the in vivo metabolism of TPO was specific and saturable, we injected normal CD-1 mice IV with trace amounts of 125I-rmTPO with or without a saturating concentration of rmTPO. The amount of radioactivity present in the spleen, blood cell fraction, platelet fraction, tibia/fibula, and femur was significantly greater in the mice receiving 125I-rmTPO alone. Conversely, the amount of radioactivity present in the plasma was significantly greater in the mice receiving both 125I-rmTPO and rmTPO, thus suggesting the uptake of rmTPO by the spleen, platelets, and bone marrow in vivo was saturable. Platelet and spleen homogenates from animals receiving 125I-rmTPO alone showed a degradation pattern of 125I-rmTPO similar to that observed in vitro using mouse platelet rich plasma. To determine the in vivo binding dynamics for rmTPO, mice were injected with 125I-rmTPO alone or with increasing concentrations of rmTPO; spleen and blood cell-associated radioactivity was determined at 2 hours postinjection. A 4-parameter curve fit of the data indicated that the "in vivo binding affinity" for rmTPO was approximately 6.4 microg/kg. These data indicate that after a dose of approximately 6.4 microg/kg, 50% of all c-Mpl receptors will be saturated with rmTPO. Electron microscopy indicated that radioactivity was present bound to and within megakaryocytes and platelets in both sternum and spleen and platelets in circulation. Together these data demonstrate that in vivo, 125I-rmTPO is mainly metabolized by platelets and to a small extent by cells of the megakaryocyte lineage, via a specific and saturable mechanism.

Human platelets as a model for the binding and degradation of thrombopoietin

Recent studies have shown that plasma thrombopoietin (TPO) levels appear to be directly regulated by platelet mass and that removal of plasma TPO by platelets via binding to the c-Mpl receptor is involved in the clearance of TPO in rodents. To help elucidate the role of platelets in the clearance of TPO in humans, we studied the in vitro specific binding of recombinant human TPO (rhTPO) to human platelet-rich plasma (PRP), washed platelets (WP), and cloned c-Mpl. Using a four-parameter fit and/or Scatchard analysis, the approximate affinity of rhTPO for its receptor, which was calculated from multiple experiments using different PRP preparations, was between 128 and 846 pmol/L, with approximately 25 to 224 receptors per platelet. WP preparations gave an affinity of 260 to 540 pmol/L, with approximately 25 to 35 receptors per platelet, and erythropoietin failed to compete with 125I-rhTPO for binding to WP. Binding and dissociation studies conducted with a BiaCore apparatus yielded an affinity of 350 pmol/L for rhTPO binding to cloned c-Mpl receptors. The ability of PRP to bind and degrade 125I-rhTPO was both time- and temperature-dependent and was blocked by the addition of excess cold rhTPO. Analysis of platelet pellets by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that 125I-rhTPO was degraded into a major fragment of approximately 45 to 50 kD. When 125I-rhTPO was incubated with a platelet homogenate at pH = 7.4, a degradation pattern similar to intact platelets was observed. Together, these data show that human platelets specifically bind rhTPO with high affinity, internalize, and then degrade the rhTPO.

Regulation of thrombopoietin levels by c-mpl-mediated binding to platelets

The involvement of platelets and the c-mpl receptor in the regulation of thrombopoietin (TPO) plasma concentrations and tissue mRNA levels was investigated in both normal mice and mice defective in c-mpl (c-mpl-/-). Although c-mpl-/- mice have fewer platelets and higher plasma TPO activity than normal mice, there was no increase in TPO mRNA levels as measured by an S1 nuclease protection assay. After the intravenous injection of 125I-TPO, specific uptake of radioactivity by the spleen and blood cells was present in the normal mice, but absent in the c-mpl-/- mice. Platelet-rich plasma (PRP) from normal mice was able to bind and internalize 125I-TPO, whereas PRP from c-mpl-/- mice lacked this ability. Analysis of 125I-TPO binding to normal PRP indicated that binding was specific and saturable, with an approximate affinity of 560 pmol/L and 220 receptors per platelet. PRP from normal mice was also able to degrade 125I-TPO into lower molecular weight fragments. After the intravenous injections, c-mpl-/- mice cleared a dose of 125I-TPO at a much slower rate than did normal mice. Injection of washed platelets from normal mice into c-mpl-/- mice resulted in a dramatic, but transient, decrease in plasma TPO levels. These data provide evidence that platelets regulate plasma TPO levels via binding to the c-mpl receptor on circulating platelets.

Characterization of the interaction of zamifenacin at muscarinic receptors in vitro

The interaction of zamifenacin ((3R)-(+)-diphenylmethoxy-1-(3,4)-methylenedioxyphenethyl)pi peridine) at muscarinic receptor subtypes was studied using radioligand binding and functional techniques, in vitro. In radioligand binding studies, zamifenacin acted as a competitive antagonist, with the following pKi values; rat cerebral cortex (M1) 7.90 +/- 0.08, myocardium (M2) 7.93 +/- 0.13, submaxillary gland (M3) 8.52 +/- 0.04 and rabbit lung (M4) 7.78 +/- 0.04. In functional studies zamifenacin acted as a surmountable antagonist, exhibiting the following apparent affinity values; canine saphenous vein (putative M1) 7.93 +/- 0.09, guinea-pig left atria (M2) 6.60 +/- 0.04, guinea-pig ileum (M3) 9.31 +/- 0.06, guinea-pig oesophageal muscularis mucosae (M3) 8.84 +/- 0.04, guinea-pig trachea (M3) 8.16 +/- 0.04, and guinea-pig urinary bladder (M3) 7.57 +/- 0.15. Therefore, zamifenacin is selective for muscarinic M3 receptors in guinea-pig ileum, oesophageal muscularis mucosae, trachea and bladder over muscarinic M2 receptors in atria. The degree of muscarinic M3/M2 receptor selectivity depends upon the muscarinic M3 receptor preparation studied.

Allosteric interactions among agonists and antagonists at 5-hydroxytryptamine3 receptors

Cooperation in the action of agonists suggests that there are multiple binding sites on 5-hydroxytryptamine3 (5-HT3) receptors. The purpose of this study was to characterize these binding sites and their interactions on both native and cloned 5-HT3 receptors. The affinities of competitive 5-HT3 receptor antagonists were similar regardless of whether the receptors were labeled with [3H]RS-42358, [3H]granisetron, or 1-(m-[3H]chlorophenyl)biguanide ([3H]mCPG). By contrast, the affinities of the agonists 5-HT, mCPG, and phenylbiguanide were approximately 10-fold higher when the receptors were labeled with [3H]mCPG. The dissociation of [3H]mCPG, [3H]RS-42358, and [3H]RS-25259, but not [3H]granisetron, from both cloned and native 5-HT3 receptors was markedly slower in the presence of 5-HT or 2-methyl-5-HT than in the presence of antagonists such as RS-42358. This suggests that the binding of these agonists to unoccupied sites on the receptor can increase the receptor's affinity for prebound ligands and thereby slow their dissociation. These data support previous indications of positive cooperation among multiple binding sites on both native and cloned 5-HT3 receptors, and they extend this idea by demonstrating that agonists can modify the interaction of some, but not all, antagonists with the receptor.

Cloning and expression of a 5-hydroxytryptamine7 receptor positively coupled to adenylyl cyclase

A cDNA clone (designated as GP2-7) encoding a novel 5-hydroxytryptamine (5-HT) receptor was isolated from a guinea pig hippocampal library. The receptor shares amino acid homology within the hydrophobic domains with other cloned 5-HT receptor subtypes (34-48%). The sequence of GP2-7 is homologous to that described for a novel receptor previously cloned from a rat brain cDNA library and provisionally designated as 5-HT7. mRNA for GP2-7 was detected in cortical and limbic brain regions. Transiently expressed GP2-7 showed high-affinity binding to [3H]5-HT (pKi = 9.0) with the following rank order of affinities: 5-carboxyamidotryptamine (5-CT) > 5-HT = 5-methoxytryptamine (5-MeOT) > methiothepin > 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) > spiperone >> sumatriptan. Adenylyl cyclase activity in CHO-K1 cells transiently transfected with GP2-7 was stimulated by several analogues of 5-HT with the following order of potency: 5-CT > 5-HT = 5-MeOT > dipropyl-5-CT > 8-OH-DPAT. Methiothepin and spiperone were potent antagonists. Preliminary analysis suggests that GP2-7 closely resembles a receptor in the guinea pig hippocampus that exhibits a high affinity toward 5-CT.

Pharmacological characterization of 5-hydroxytryptamine3 receptors in murine brain and ileum using the novel radioligand [3H]RS-42358-197: evidence for receptor heterogeneity

Previous studies have demonstrated species-specific differences in 5-hydroxytryptamine3 (5-HT3) receptors, but unequivocal evidence of 5-HT3 receptor subtypes, within a species, has not yet been obtained. The purpose of the current study was to test for heterogeneity in 5-HT3 receptors in murine tissues. 5-HT3 receptors in membranes derived from brain cerebral cortex of CD-1, C57Bl/6, and Swiss Webster mice and ileum of CD-1 mice were labeled with the 5-HT3 receptor antagonist [3H]RS-42358-197. Structurally diverse competing ligands were then used to characterize the binding site. [3H]RS-42358-197 bound with similar affinity in each of the cortical tissues (mean KD = 0.14 nM; range, 0.06-0.32 nM) but bound with lower affinity in ileal tissue (2.5 nM). The density of sites labeled with [3H]RS-42358-197 ranged from 10.4 fmol/mg of protein in Swiss Webster mouse cortex to 44.2 fmol/mg of protein in Sprague-Dawley rat cortex. Displacing ligands produced a pharmacologic profile of the [3H]RS-42358-197 binding site consistent with it being a 5-HT3 receptor: (R)-YM060 > (S)-zacopride > (R)-zacopride > MDL 72222 > 2-methyl-5-HT. However, > or = 10-fold differences in the affinity of certain ligands were found when comparing 5-HT3 binding sites in membranes from cerebral cortex of the different strains of mice and when comparing 5-HT3 binding sites in brain and ileal membranes prepared from the CD-1 mouse strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Labelling of 5-hydroxytryptamine3 receptors with a novel 5-HT3 receptor ligand, [3H]RS-42358-197

RS-42358-197[(S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1 H-benzo[de]isoquinolin-1-one hydrochloride] displaced the prototypic 5-hydroxytryptamine3 (5-HT3) receptor ligand [3H]quipazine in rat cerebral cortical membranes with an affinity (pKi) of 9.8 +/- 0.1, while having weak affinity (pKi < 6.0) in 23 other receptor binding assays. [3H]RS-42358-197 was then utilized to label 5-HT3 receptors in a variety of tissues. [3H]RS-42358-197 labelled high-affinity and saturable binding sites in membranes from rat cortex, NG108-15 cells, and rabbit ileal myenteric plexus with affinities (KD) of 0.12 +/- 0.01, 0.20 +/- 0.01, and 0.10 +/- 0.01 nM and densities (Bmax) of 16.0 +/- 2.0, 660 +/- 74, and 88 +/- 12 fmol/mg of protein, respectively. The density of sites labelled in each of these tissues with [3H]RS-42358-197 was similar to that labelled with [3H]GR 65630, but was significantly less than that found with [3H]-quipazine. The binding of [3H]RS-42358-197 had a pharmacological profile similar to that of [3H]quipazine, as indicated by the rank order of displacement potencies: RS-42358-197 > (S)-zacopride > tropisetron > (R)-zacopride > ondansetron > MDL72222 > 5-HT. However, differences in 5-HT3 receptors of different tissues and species were detected on the basis of statistically significant differences in the affinities of phenylbiguanide, and 1-(m-chlorophenyl)biguanide when displacing [3H]RS-42358-197 binding. [3H]RS-42358-197 also labelled a population (Bmax = 91 +/- 17 fmol/mg of protein) of binding sites in guinea pig myenteric plexus membranes, with lower affinity (KD = 1.6 +/- 0.3 nM) than those in the other preparations. Moreover, the rank order of displacement potencies of 15 5-HT3 receptor ligands in guinea pig ileum was found not to be identical to that in other tissues. Binding studies carried out with [3H]RS-42358-197 have detected differences in 5-HT3 receptor binding sites in tissues of different species and further underscore the unique nature of the guinea pig 5-HT3 receptor.

Characterization of the interaction of the cervane alkaloid, imperialine, at muscarinic receptors in vitro

The action of the cervane alkaloid, imperialine, has been assessed at M1, M2 and M3 receptors in functional assays and at M1, M2, M3 and putative M4 sites in binding studies. In functional studies, imperialine acted as a selective surmountable antagonist at M2 receptors in guinea-pig isolated atria and uterus (-log KB = 7.7 and 7.4, respectively), in comparison to M1 receptors in canine isolated saphenous vein (-log KB = 6.9) or M3 receptors in a range of guinea-pig isolated smooth muscles including ileum, trachea, fundus, seminal vesicle or oesophagus (-log KB = 6.6-6.8). In rat aorta, the -log KB value at the M3 receptor (5.9) was slightly, but significantly, lower. In competition radioligand binding studies, imperialine was also selective toward to M2 sites in rat myocardium (-log Ki = 7.2) with respect to M1 and M3 sites (rat cerebral cortex, rat submaxillary gland; -log Ki = 6.1 and 5.7, respectively). However, it did not significantly discriminate between rat cardiac M2 sites and putative M4 sites in rabbit lung (-log Ki = 6.9). Imperialine resembles the alkaloid himbacine in terms of its pharmacological profile at muscarinic receptor subtypes in that it acts as an M2 selective antagonist with respect to M1 or M3 sites. It may also provide a second, commercially available, antagonist with which to discriminate between M1 and M4 receptors.

Characteristics of 5-HT3 binding sites in NG108-15, NCB-20 neuroblastoma cells and rat cerebral cortex using [3H]-quipazine and [3H]-GR65630 binding

1. The biochemical and pharmacological properties of 5-HT3 receptors in homogenates of NG108-15 and NCB-20 neuroblastoma cells and rat cerebral cortex have been ascertained by the use of [3H]-quipazine and [3H]-GR65630 binding. 2. In NG108-15 and NCB-20 cell homogenates, [3H]-quipazine bound to a single class of high affinity (NG108-15: Kd = 6.2 +/- 1.1 nM, n = 4; NCB-20: Kd = 3.0 +/- 0.9 nM, n = 4; means +/- s.e.means) saturable (NG108-15: Bmax = 1340 +/- 220 fmol mg-1 protein; NCB-20: Bmax = 2300 +/- 200 fmol mg-1 protein) binding sites. In rat cortical homogenates, [3H]-quipazine bound to two populations of binding sites in the absence of the 5-hydroxytryptamine (5-HT) uptake inhibitor, paroxetine (Kd1 = 1.6 +/- 0.5 nM, Bmax1 = 75 +/- 14 fmol mg-1 protein; Kd2 = 500 +/- 300 nM, Bmax2 = 1840 +/- 1040 fmol mg-1 protein, n = 3), and to a single class of high affinity binding sites (Kd = 2.0 +/- 0.5 nM, n = 3; Bmax = 73 +/- 6 fmol mg-1 protein) in the presence of paroxetine. The high affinity (nanomolar) component probably represented 5-HT3 binding sites and the low affinity component represented 5-HT uptake sites. 3. [3H]-paroxetine bound with high affinity (Kd = 0.02 +/- 0.003 nM, n = 3) to a site in rat cortical homogenates in a saturable (Bmax = 323 +/- 45 fmol mg-1 protein, n = 3) and reversible manner. Binding to this site was potently inhibited by 5-HT uptake blockers such as paroxetine and fluoxetine (pKi s = 8.6-9.9), while 5-HT3 receptor ligands exhibited only low affinity (pK; < 7). No detectable specific [3H]-paroxetine binding was observed in NG108-15 or NCB-20 cell homogenates. 4. [3H]-quipazine binding to homogenates of NG108-15, NCB-20 cells and rat cortex (in the presence of 0.1 microM paroxetine) exhibited similar pharmacological characteristics. 5-HT3 receptor antagonists competed for [3H]-quipazine binding with high nanomolar affinities in the three preparations and the rank order of affinity was: (S)-zacopride > quarternized ICS 205-930 2 granisetron > ondansetron > ICS 205-209 (R)-zacopride > quipazine > renzapride > MDL-72222 > butanopride > metoclopramide. 5. [3H]-GR65630 labelled a site in NCB-20 cell homogenates with an affinity (Kd = 0.7 + 0.1 nms n = 4) and density (B__ = 1800 + 1000 fmol mg- protein) comparable to that observed with [3H]-quipazine. Competition studies also indicated a good correlation between the pharmacology of 5-HT3 binding sites when [3H]-GR65630 and [3H]-quipazine were used in these cells. 6. In conclusion, [3H]-quipazine labelled 5-HT3 receptor sites in homogenates of NG108-15 cells, NCB-20 cells and rat cerebral cortex. In rat cortical homogenates, [3H]-quipazine also bound to 5-HT uptake sites, which could be blocked by 0.1 microM paroxetine. The pharmacological specificity of the 5-HT3 receptor labelled by [3H]-quipazine was similar in the neuroblastoma cells and rat cortex and was substantiated in NCB-20 cells by the binding profile of the selective 5-HT3 receptor antagonist, [3H]-GR65630.

Direct labeling of rat M3-muscarinic receptors by [3H]4DAMP

The muscarinic receptors of rat submaxillary gland, rat heart and rat cortex were directly labeled using the ligand [3H]4-diphenylacetoxy-N-methyl-piperidine methiodide [( 3H]4DAMP). In the rat submaxillary gland, [3H]4DAMP predominantly bound with high affinity (Kd = 0.2 nM) to a population of binding sites that displayed the pharmacology of the M3 muscarinic receptor subtype. In rat heart, [3H]4DAMP labeled the M2 muscarinic receptor with low affinity (Kd = 4 nM). In rat cortex [3H]4DAMP predominantly bound to a population of sites with high affinity (Kd = 0.2 nM). The pharmacology of these sites was consistent with [3H]4DAMP labeling both M1 and M3 muscarinic receptors present in rat cortex with high affinity. These data indicate that [3H]4DAMP represents a useful ligand for selectively labeling the M1 and M3 muscarinic receptor subtypes.

Binding characteristics of the muscarinic receptor subtype of the NG108-15 cell line

Kinetic, saturation and competition binding studies were conducted on the muscarinic receptor binding sites labeled by [3H]N-methylscopolamine ([3H]NMS) in membranes prepared from NG108-15 cells. The pharmacology of the NG108-15 cell muscarinic receptors was compared to that of the M1 receptors of rat cortex labeled using [3H]pirenzepine, the M2 and M3 receptors of rat heart and submaxillary gland, respectively, labeled using [3H]NMS and the muscarinic receptors of the PC12 cell line also labeled using [3H]NMS. The rate of dissociation of [3H]NMS from the NG108-15 cell muscarinic receptor was similar to that obtained at the M3 receptor and at the muscarinic receptor of the P12 cells but was slower that the dissociation rate obtained at the M2 cardiac muscarinic receptor. The Kd of [3H]NMS in the NG108-15 cells was significantly lower than that obtained at the M2 and M3 receptor but was similar to the Kd obtained in PC12 cells. In competition studies the affinity estimates for AF-DX 116, 4-DAMP, methoctramine and pirenzepine were not consistent with the presence of either an M1, M2 or M3 receptor but were identical to the affinity estimates obtained at the muscarinic receptor of the PC12 cell line. On the basis of these data we conclude that the muscarinic receptor present in the NG108-15 cells is different to the M1, M2 or M3 subtypes already described but is similar to the muscarinic receptor present in the PC12 cell line. Since NG108-15 cells expresses mRNA for the m4 muscarinic receptor gene described by Bonner et al. (1987) we propose that the muscarinic receptors present in this cell line be denoted as M4 receptors.

PC12 phaeochromocytoma cells contain an atypical muscarinic receptor binding site

1. Kinetic, saturation and competition binding studies were conducted on the muscarinic receptor binding site labelled by [3H]-N-methylscopolamine [( 3H]-NMS) in intact PC12 cells and cell membranes. Similar studies were conducted on M1 receptors of rat cortex labelled with [3H]-pirenzepine and M2 and M3 receptors present in rat heart and submaxillary gland respectively, and labelled with [3H]-NMS. 2. The dissociation of [3H]-NMS from muscarinic receptors in PC12 cells was slower than dissociation from both M2 and M3 muscarinic receptors. 3. The Kd of [3H]-NMS in the PC12 cells was significantly lower than that obtained at the M2 and M3 receptor. 4. In competition studies the affinity data for pirenzepine, hexahydroadiphenine and 4-diphenylacetoxy-N-methylpiperidine methiodide were consistent with the presence of an M3 receptor in the PC12 cells. However, for AF-DX 116, cyclohexylphenyl(2-piperidinoethyl)silanol and methoctramine affinity estimates in PC12 cells were 3-6 fold lower than at the M3 receptor. 5. On the basis of these data we conclude that the muscarinic receptor present in the PC12 cells differs from the M1, M2 and M3 subtypes already described.