Simultaneous determination of FL118 and W34 in rat Blood by LC-MS/MS: Application to pharmacokinetic studies.

W34 is a prodrug of FL118, and it can be converted to FL118 via a hydrolysis reaction. In this report, a highly sensitive LC-MS/MS method using a C18 column was validated and used for the simultaneous determination of W34 and FL118 in rat blood. A stepwise gradient elution with 0.1% formic acid in water and acetonitrile was employed. The assays were linear over a concentration range of 0.50-50.0 ng/ml for both W34 and FL118. The accuracy of the validation method ranged from 89.74 to 98.94% for W34 and from 88.61 to 94.60% for FL118. The precision was within 7.15% for W34 and 9.63% for FL118. Extraction recoveries of W34 were 94.56-100.49 and 87.67-106.32% for FL118. No significant matrix effects for both W34 and FL118 were observed in blood. The assay has been successfully applied to biological samples obtained from a stability and pharmacokinetic study of W34 and FL118.

Drug likeness, targets, molecular docking and ADMET studies for some indolizine derivatives.

The aim of this work was to investigate the biomolecular targets for a library of indolizines, study their molecular properties, drug likeness, target prediction, performing docking studies and exploring their ADMET profile in search for a lead compound. All compounds appeared to comply with Lipinski's rule without any violation, additionally their solubility is viewed good except for compounds 4a-c which are anticipated to be reasonably soluble, their Milog P score was 4.18-4.9, proposing that these compounds are the most lipophilic with least water solubility, however this may be helpful as the cannabinoid receptor-1 is the most probable target for these three compounds. The inclusive target for the selected library was tau protein. Structure based studies demonstrated great fitting of indolizines with tau protein, along these lines they are expected to have pharmacological action in vivo. This urged us to think about the ADMET properties of this library. These investigations suggested the ability of the selected compounds to pass the blood brain barrier (BBB) (aside from them compounds 2c and 3c) and affect tau proteins, which will be valuable for the treatment of Alzheimer's disease, particularly compound 5 which does not require any SAR modifications to attain the BBB.

sp2 -Iminosugar α-glucosidase inhibitor 1-C-octyl-2-oxa-3-oxocastanospermine specifically affected breast cancer cell migration through Stim1, ß1-integrin, and FAK signaling pathways.

Aberrant glycosylation changes on many glycoproteins are often related to cancer progression and metastasis. sp2 -Iminosugar-type castanospermine analogues, inhibitors of α-glucosidases, have been reported to exhibit antitumor activity. However, their effects on cell migration and the underlying molecular mechanism are not fully understood. Here, we investigated the effect of the pseudo-C-octyl glycoside 2-oxa-3-oxocastanospermine derivatives (CO-OCS) on breast cancer cells (MCF-7 and MDA-MB-231 cells), and MCF-10A mammary normal cell lines. We showed that CO-OCS treatment results in the drastic decrease of breast cancer cell migration without affecting cell proliferation. Furthermore, CO-OCS significantly reduced both the expression of ß1-integrin, which is a crucial interacting partner of Focal Adhesion Kinase (FAK), and the phosphorylation rates of FAK and ERK1/2. CO-OCS also drastically reduced Ca2+ entry through Store Operated Channels (SOC). Orai1 and Stim1, two N-glycosylated proteins, are involved in Store-Operated Calcium Entry (SOCE), and are essential for breast tumor cell migration. Our results showed that CO-OCS decreased the expression, at the protein level, of Stim1 without affecting that of Orai1. Moreover, cell migration and SOCE were attenuated by CO-OCS as well as when Stim1 was silenced. In contrast, in MCF-10A cells, CO-OCS slightly reduced cell migration, but was without effect on gene expression of Stim1, Orai1, ß1-integrin, or FAK and ERK1/2 activation. Our results provide strong evidence for a significant effect of CO-OCS on breast cancer cell migration and support that this effect was associated with ß1-integrin, Stim1, and FAK signaling pathways.

Air flow-assisted ionization imaging mass spectrometry method for easy whole-body molecular imaging under ambient conditions.

Whole-body molecular imaging is able to directly map spatial distribution of molecules and monitor its biotransformation in intact biological tissue sections. Imaging mass spectrometry (IMS), a label-free molecular imaging method, can be used to image multiple molecules in a single measurement with high specificity. Herein, a novel easy-to-implement, whole-body IMS method was developed with air flow-assisted ionization in a desorption electrospray ionization mode. The developed IMS method can effectively image molecules in a large whole-body section in open air without sample pretreatment, such as chemical labeling, section division, or matrix deposition. Moreover, the signal levels were improved, and the spatial assignment errors were eliminated; thus, high-quality whole-body images were obtained. With this novel IMS method, in situ mapping analysis of molecules was performed in adult rat sections with picomolar sensitivity under ambient conditions, and the dynamic information of molecule distribution and its biotransformation was provided to uncover molecular events at the whole-animal level. A global view of the differential distribution of an anticancer agent and its metabolites was simultaneously acquired in whole-body rat and model mouse bearing neuroglioma along the administration time. The obtained drug distribution provided rich information for identifying the targeted organs and predicting possible tumor spectrum, pharmacological activity, and potential toxicity of drug candidates.

Discovery and characterization of NVP-QAV680, a potent and selective CRTh2 receptor antagonist suitable for clinical testing in allergic diseases.

Optimization of a 7-azaindole-3-acetic acid CRTh2 receptor antagonist chemotype derived from high throughput screening furnished a highly selective compound NVP-QAV680 with low nM functional potency for inhibition of CRTh2 driven human eosinophil and Th2 lymphocyte activation in vitro. The molecule exhibited good oral bioavailability in the rat, combined with efficacy in rodent CRTh2-dependent mechanistic and allergic disease models and was suitable for clinical development.

Tetrahydroindolizinone NK1 antagonists.

A new class of potent NK(1) receptor antagonists with a tetrahydroindolizinone core has been identified. This series of compounds demonstrated improved functional activities as compared to previously identified 5,5-fused pyrrolidine lead structures. SAR at the 7-position of the tetrahydroindolizinone core is discussed in detail. A number of compounds displayed high NK(1) receptor occupancy at both 1 h and 24 h in a gerbil foot tapping model. Compound 40 has high NK(1) binding affinity, good selectivity for other NK receptors and promising in vivo properties. It also has clean P(450) inhibition and hPXR induction profiles.

Comparative disposition and metabolism of paraherquamide in sheep, gerbils, and dogs.

The disposition and metabolism of paraherquamide (PHQ), a potent and broad-spectrum anthelminthic, were examined in sheep, dogs, and gerbils. The metabolism of PHQ in these species was extensive and marked by significant species differences both in vitro and in vivo. In sheep and gerbils, PHQ metabolism occurs mainly at the pyrrolidine moiety, generating several metabolites that, for the most part, retained nematodicidal activity in vitro. In dogs, the dioxepene group was also extensively metabolized, ultimately resulting in formation of a catechol and loss of pharmacological activity. After oral administration of [3H]PHQ to intact sheep, gerbils, and dogs, the majority of the administered radioactivity was recovered in feces. Intact PHQ accounted for 0% (dogs) to approximately 30% (sheep and gerbils) of drug-related material in feces. A detailed investigation of the composition of the intestinal content of sheep indicated that a significant amount of the dose was still present in the rumen 24 h after dose and that PHQ underwent significant dehydration in the cecum. The oral pharmacokinetic parameters of PHQ in sheep and dogs suggest that its absorption is rapid in both species but that its apparent elimination rate is significantly higher in the dog (t(1/2) approximately 1.5 h) than it is in sheep (t(1/2) approximately 8.5 h). The short elimination half-life and the absence of PHQ or other active components in the dog gastrointestinal tract provide a potential explanation of the lack of efficacy of PHQ in this species.

A comparison study between lycobetaine-loaded nanoemulsion and liposome using nRGD as therapeutic adjuvant for lung cancer therapy.

To achieve tumor-selective drug delivery, various nanocarriers have been explored using either passive or active targeting strategies. Despite the great number of studies published annually in the field, only nanocarriers using approved excipients reach the clinical stage. In our study, two classic nanoscale formulations, nanoemulsion (NE) and liposome (Lipo) were selected for the encapsulation of lycobetaine (LBT). To improve the lipid solubility of LBT, oleic acid (OA) was used to complex (LBT-OA) with lycobetaine (LBT). Besides, PEGylated lecithin was used to enhance the circulation time. The release behaviors of LBT from non-PEGylated and PEGylated NE and Lipo were compared. PEGylated LBT-OA loaded Lipo (LBT-OA-PEG-Lipo) exhibited a sustained release rate pattern, and in vivo pharmacokinetic profiles showed the extended circulation compared nanoemlusions. Besides, LBT-OA-PEG-Lipo showed an enhanced anti-tumor effect in the mice xenograft lung carcinoma model. Moreover, a multi-target peptide nRGD was co-administered as a therapeutic adjuvant with LBT-OA loaded formulations, which demonstrated improved tumor penetration and enhanced extravasation of formulations. Also, co-administration of nRGD significantly improved the in vivo antitumor efficacy of different formulations, likely due to the depletion of tumor-associated macrophages (TAMs). Thus, LBT-OA-PEG-Lipo+nRGD may represent a promising strategy for cancer chemotherapy against lung carcinoma.

Analysis of Lycobetaine in Rat Plasma by LC-ESI-MS/MS.

In this study, a selective and sensitive liquid chromatography-electrospray ionization-tandem mass spectrometric method was developed and validated for the determination of lycobetaine in rat plasma. Berberine was selected as the internal standard, and rat plasma samples were pretreated via protein precipitation and further separated on a diamonsil octadecyl-silylated silica column using 0.2% (v/v) aqueous formic acid and methanol as the mobile phase. Selected reaction monitoring was performed using the transitions m/z 266.1 → 208.1 and m/z 336.1 → 320.0 to determine the concentrations of lycobetaine and internal standard, respectively. The injection volume was 1 µL, and the calibration curve was linear (R2 = 0.9998), while the validated lower limit of quantification was 25 ng/mL. Precision varied from 3.4% to 9.9%, and accuracy varied from -2.6% to 8.7%. Lycobetaine remained stable under all relevant analytical conditions tested in the study. The method was successfully applied to determine the plasma concentration of lycobetaine in a pharmacokinetic study. After intravenous administration of 10 mg/kg and oral administration of 200 mg/kg lycobetaine in rats, the pharmacokinetic parameters were calculated and the oral bioavailability of lycobetaine was determined as 7.30% ± 1.44%.

Discovery and Characterization of GSK2801, a Selective Chemical Probe for the Bromodomains BAZ2A and BAZ2B.

Bromodomains are acetyl-lysine specific protein interaction domains that have recently emerged as a new target class for the development of inhibitors that modulate gene transcription. The two closely related bromodomain containing proteins BAZ2A and BAZ2B constitute the central scaffolding protein of the nucleolar remodeling complex (NoRC) that regulates the expression of noncoding RNAs. However, BAZ2 bromodomains have low predicted druggability and so far no selective inhibitors have been published. Here we report the development of GSK2801, a potent, selective and cell active acetyl-lysine competitive inhibitor of BAZ2A and BAZ2B bromodomains as well as the inactive control compound GSK8573. GSK2801 binds to BAZ2 bromodomains with dissociation constants (KD) of 136 and 257 nM for BAZ2B and BAZ2A, respectively. Crystal structures demonstrated a canonical acetyl-lysine competitive binding mode. Cellular activity was demonstrated using fluorescent recovery after photobleaching (FRAP) monitoring displacement of GFP-BAZ2A from acetylated chromatin. A pharmacokinetic study in mice showed that GSK2801 had reasonable in vivo exposure after oral dosing, with modest clearance and reasonable plasma stability. Thus, GSK2801 represents a versatile tool compound for cellular and in vivo studies to understand the role of BAZ2 bromodomains in chromatin biology.

Extended Evaluation of Virological, Immunological and Pharmacokinetic Endpoints of CELADEN: A Randomized, Placebo-Controlled Trial of Celgosivir in Dengue Fever Patients.

UNLABELLED: CELADEN was a randomized placebo-controlled trial of 50 patients with confirmed dengue fever to evaluate the efficacy and safety of celgosivir (A study registered at ClinicalTrials.gov, number NCT01619969). Celgosivir was given as a 400 mg loading dose and 200 mg bid (twice a day) over 5 days. Replication competent virus was measured by plaque assay and compared to reverse transcription quantitative PCR (qPCR) of viral RNA. Pharmacokinetics (PK) correlations with viremia, immunological profiling, next generation sequence (NGS) analysis and hematological data were evaluated as exploratory endpoints here to identify possible signals of pharmacological activity. Viremia by plaque assay strongly correlated with qPCR during the first four days. Immunological profiling demonstrated a qualitative shift in T helper cell profile during the course of infection. NGS analysis did not reveal any prominent signature that could be associated with drug treatment; however the phylogenetic spread of patients' isolates underlines the importance of strain variability that may potentially confound interpretation of dengue drug trials conducted during different outbreaks and in different countries. Celgosivir rapidly converted to castanospermine (Cast) with mean peak and trough concentrations of 5727 ng/mL (30.2 µM) and 430 ng/mL (2.3 µM), respectively and cleared with a half-life of 2.5 (± 0.6) hr. Mean viral log reduction between day 2 and 4 (VLR2-4) was significantly greater in secondary dengue than primary dengue (p = 0.002). VLR2-4 did not correlate with drug AUC but showed a trend of greater response with increasing Cmin. PK modeling identified dosing regimens predicted to achieve 2.4 to 4.5 times higher Cmin. than in the CELADEN trial for only 13% to 33% increase in overall dose. A small, non-statistical trend towards better outcome on platelet nadir and difference between maximum and minimum hematocrit was observed in celgosivir-treated patients with secondary dengue infection. Optimization of the dosing regimen and patient stratification may enhance the ability of a clinical trial to demonstrate celgosivir activity in treating dengue fever based on hematological endpoints. A new clinical trial with a revised dosing regimen is slated to start in 2016 (NCT02569827). Furthermore celgosivir's potential value for treatment of other flaviruses such as Zika virus should be investigated urgently. TRIAL REGISTRATION: ClinicalTrials.gov NCT01619969.

Characterization of the binding of [3H]SR 33805 to the slow Ca2+ channel in rat heart sarcolemmal membrane.

SR 33805 is a representative of a new class of compounds (indole sulfone) that inhibits L-type Ca2+ channels. [3H]SR 33805 has been shown to bind with a high affinity (Kd approximately 20 pM calculated from saturation isotherms and association/dissociation kinetics) to a single site in a purified preparation of rat cardiac sarcolemmal membranes. The binding was found to be saturable and reversible. The maximal binding capacity was in approximately 1:1 stoichiometry with that of other Ca2+ channel antagonists. Various cations (Na+, Ca2+, Cd2+, and La3+) were shown to inhibit specific [3H]SR 33805 binding, with La3+ being the most potent. Using a range of receptor or channel ligands (including omega-conotoxin and Na+ and K+ channel modulators), only the L-type Ca2+ channel antagonists were found to displace [3H]SR 33805. However, dihydropyridines, phenylalkylamines, benzothiazepines, and diphenyl-butylpiperidines were found to inhibit [3H]SR 33805 in a non-competitive manner as demonstrated by displacement experiments in addition to dissociation kinetics. In contrast, the interaction of SR 33805 with fantofarone has been found to be competitive. Binding of [3H]SR 33805 (and [3H]fantofarone) is entropy driven as opposed to that of the [3H]nitrendipine which is enthalpy driven. From these results we suggest that SR 33805 binds with a high affinity to a unique site on the L-type Ca2+ channel found in rat cardiac sarcolemmal membranes. This site is equivalent to that of fantofarone and in allosteric interaction with that of the dihydropyridines, phenylalkylamines and benzothiazepines.

Evaluation of the excretion and metabolism of the new analgesic agent RWJ-22757 in male and female CR Wistar rats.

The excretion and metabolism of (+/-)-trans-3-(2-bromophenyl)octahydroindolizine hydrochloride (RWJ-22757) have been investigated in male and female CR Wistar rats. Radiolabeled [14C] RWJ-22757 was administered orally to each of the rats as a single 60 mg/kg suspension dose. Plasma (0-48 h), urine (0-168 h) and fecal (0-168 h) samples were collected and analyzed. There were no significant gender differences observed in the data. The estimated elimination half-life of the total radioactivity from plasma was 19 h while the estimated elimination half-life of RWJ-22757 was 15 h. Recoveries of total radioactivity in urine and feces were 58.4+/-5.8 and 42.4+/-6.3%, respectively. RWJ-22757 and a total of 11 metabolites were isolated in rat plasma, urine, and fecal extracts. The structures of four of these metabolites were tentatively identified. Unchanged RWJ-22757 accounted for < 4% of the dose in plasma and urine and 28% in feces; thus, indicating the drug was extensively metabolized and either not absorbed well or biliary excreted. Identified metabolites accounted for > 80% of the total radioactivity contained in the samples. The following pathways were used to describe the formation of the metabolites identified in rats: octahydroindolizine ring oxidation, phenyl hydroxylation, octahydroindolizine ring oxidation followed by ring opening to a carboxylic acid function and octahydroindolizine ring oxidation followed by ring opening and N-methylation.

BUCAST: another new antiviral.

AIDS: BUCAST is a butanoyl derivative of castanospermine, a naturally occurring plant alkaloid. BUCAST has been shown to make HIV less infectious and pathogenic by changing the sugar molecules of gp160. Test-tube experiments show BUCAST to have activity against both laboratory and clinical isolates of HIV and to be synergistic with AZT, ddC, and ddI. Maximum tolerated doses appear to be 400 mg per day, above which severe toxicities occur. BUCAST is rapidly absorbed when taken orally and is eliminated from the body in about 72 hours without appearing to accumulate. Several efficacy studies involving BUCAST are enrolling at sites in Kansas City, Fort Lauderdale, and San Francisco.^ieng

Nanoemulsion loaded with lycobetaine-oleic acid ionic complex: physicochemical characteristics, in vitro, in vivo evaluation, and antitumor activity.

BACKGROUND: Intravenous injection of lycobetaine was found to show significant cytotoxic activity against (inter alia) Lewis lung carcinoma, but its therapeutic use is largely limited due to an extremely short half-life in blood. This study aimed at developing a novel lipid nanocarrier-based formulation for lycobetaine delivery. The formulation is feasible for scale-up production, exhibiting good parenteral acceptability and improved circulation characteristics. METHODS: To enhance its lipophilicity, oleic acid was selected to form ionic complexes with lycobetaine (LBT). The nanoemulsion loaded with LBT-oleic acid complex (LBT-OA-nanoemulsion) and PEGylated LBT-OA-nanoemulsion (NE) (LBT-OA-PEG-NE) were prepared by a simple high-pressure homogenization method. RESULTS: A high-encapsulation efficiency of around 97.32% ± 2.09% was obtained for LBT-OA-PEG-NE under optimized conditions. Furthermore, the in vivo pharmacokinetics and biodistribution of LBT-OA-NE, LBT-OA-PEG-NE, and free LBT were studied in rats. Free LBT and LBT-OA-PEG-NE displayed AUC0-10h (area under the concentration-time curve from 0 to 10 hours) of 112.99 mg/L*minute and 3452.09 mg/L*minute via intravenous administration (P < 0.005), respectively. Moreover, LBT-OA-PEG-NE showed significantly lower LBT concentration in the heart, liver, and kidney, while achieving higher concentration of LBT in the lung when compared to free LBT at the same time (P < 0.005). The LBT-OA-PEG-NE exhibited higher growth inhibitory effect and longer survival time than free LBT in both heterotopic and lung metastatic tumor models. CONCLUSION: These results demonstrated that LBT-OA-PEG-NE is an attractive parenteral formulation for cancer therapy.

Dose- and schedule-dependent protective efficacy of celgosivir in a lethal mouse model for dengue virus infection informs dosing regimen for a proof of concept clinical trial.

Celgosivir (6-O-butanoyl castanospermine), a pro-drug of the naturally occurring castanospermine, is an inhibitor of α-glucosidase I and II that is found to be a potent inhibitor of several enveloped viruses including all four serotypes of dengue virus. We showed previously that the compound fully protected AG129 mice from lethal infection with a mouse adapted dengue virus at a dose of 50mg/kg twice daily (BID) for 5days and was effective even after 48h delayed treatment. Here we show that the protection by celgosivir is dose- and schedule-dependent and that a twice-a-day regimen of 50, 25 or 10mg/kg is more protective than a single daily dose of 100mg/kg. Treatment with 50mg/kg BID castanospermine had comparable efficacy as 25mg/kg BID celgosivir, suggesting that celgosivir is approximately twice as potent as castanospermine with respect to in vivo antiviral efficacy. Pharmacokinetics (PK) studies of celgosivir in mice showed that it rapidly metabolized to castanospermine. Simulation of the PK data with the survival data for the various doses of celgosivir tested suggests that the steady-state minimum concentration is a critical parameter to note in choosing dose and schedule. These results influenced the selection of the dose regimen for a proof-of-concept clinical trial of celgosivir as a treatment against dengue fever.

Synthesis and biological evaluation of tylophorine-derived dibenzoquinolines as orally active agents: exploration of the role of tylophorine e ring on biological activity.

A series of novel tylophorine-derived dibenzoquinolines has been synthesized and their biological activity evaluated. Three assays were conducted: inhibition of cancer cell proliferation, inhibition of TGEV replication for anticoronavirus activity, and suppression of nitric oxide production in RAW264.7 cells (a measure of anti-inflammation). The most potent compound from these assays, dibenzoquinoline 33b, showed improved solubility compared to tylophorine 9a, in vivo efficacies in a lung A549 xenografted tumor mouse model and a murine paw edema model, good bioavailability, and no significant neurotoxicity (as tested by a rota-rod test for motor coordination). This is the first study to explore in detail the role of the tylophorine E ring on biological activity and very strongly suggests that tylophorine-derived dibenzoquinolines merit further development into orally active agents.

Identification of phenanthroindolizines and phenanthroquinolizidines as novel potent anti-coronaviral agents for porcine enteropathogenic coronavirus transmissible gastroenteritis virus and human severe acute respiratory syndrome coronavirus.

The discovery and development of new, highly potent anti-coronavirus agents and effective approaches for controlling the potential emergence of epidemic coronaviruses still remains an important mission. Here, we identified tylophorine compounds, including naturally occurring and synthetic phenanthroindolizidines and phenanthroquinolizidines, as potent in vitro inhibitors of enteropathogenic coronavirus transmissible gastroenteritis virus (TGEV). The potent compounds showed 50% maximal effective concentration (EC50) values ranging from 8 to 1468 nM as determined by immunofluorescent assay of the expression of TGEV N and S proteins and by real time-quantitative PCR analysis of viral yields. Furthermore, the potent tylophorine compounds exerted profound anti-TGEV replication activity and thereby blocked the TGEV-induced apoptosis and subsequent cytopathic effect in ST cells. Analysis of the structure-activity relations indicated that the most active tylophorine analogues were compounds with a hydroxyl group at the C14 position of the indolizidine moiety or at the C3 position of the phenanthrene moiety and that the quinolizidine counterparts were more potent than indolizidines. In addition, tylophorine compounds strongly reduced cytopathic effect in Vero 76 cells induced by human severe acute respiratory syndrome coronavirus (SARS CoV), with EC50 values ranging from less than 5 to 340 nM. Moreover, a pharmacokinetic study demonstrated high and comparable oral bioavailabilities of 7-methoxycryptopleurine (52.7%) and the naturally occurring tylophorine (65.7%) in rats. Thus, our results suggest that tylophorine compounds are novel and potent anti-coronavirus agents that may be developed into therapeutic agents for treating TGEV or SARS CoV infection.

Celgosivir, an alpha-glucosidase I inhibitor for the potential treatment of HCV infection.

Celgosivir, in development by MIGENIX Inc for the treatment of HCV infection, is an oral prodrug of the natural product castanospermine that inhibits alpha-glucosidase I, an enzyme that plays a critical role in viral maturation by initiating the processing of the N-linked oligosaccharides of viral envelope glycoproteins. Celgosivir is well absorbed in vitro and in vivo, and is rapidly converted to castanospermine. Celgosivir has a novel mechanism of action (ie, host-directed glycosylation), and demonstrates broad antiviral activity in vitro. The agent is not efficient as a monotherapy for the treatment of HCV, but has demonstrated a synergistic effect in combination with the current standard of care, PEGylated IFNalpha2b plus ribavirin, both in vitro and in phase II clinical trials. At the time of publication, a phase II trial was underway to investigate the safety, tolerability and antiviral effect of celgosivir in combination with PEGylated IFNalpha2b plus ribavirin for up to 1 year in patients with chronic HCV infection. Celgosivir may prove to be a valuable component for combination therapy and may help to prevent the apparition of drug resistance. Long-term toxicity studies are necessary to confirm the safety of this novel drug in humans.

Assessment of basic indirect pharmacodynamic response models with physiological limits.

Many physiological factors are regulated by homeostatic mechanisms to maintain normal body function. Empirical lower Rl (Model I and IV) or upper Rh limits (Model II and III) were included in current basic indirect response (IDR) models to account for the additional role of physiological limits (IDRPL). Various characteristics of these models were evaluated with simulations and explicit equations. The simulations reveal that the expanded models exhibit most properties of basic indirect response models, such as slow response initiation, lag time between the kinetic and dynamic peaks, a large dose plateau, and shift in Tmax with dose. The proposed models always produce lesser net responses than predicted by basic IDR models. Simulations demonstrate that addition of a parameter limit which is close to the baseline has a great influence on the overall and maximum responses and fitted model parameters. Only stimulatory IDRPL Models III and IV allow resolution of all model parameters in the absence of clear indications or predetermined values of the lower or upper limits. However, all four models are able to resolve model parameters when subgroups with different baselines are simultaneously fitted. These models create new interpretations of the determinants of baseline conditions which can be important in assessing inter-subject variability in responses. The applicability of IDRPL models is demonstrated using several examples from the published literature. Indirect response models with physiological limits will be useful in characterizing drug responses for turnover systems which are maintained within a certain range.