Emil T. Lin

3-Iodothyronamine is an endogenous and rapid-acting derivative of thyroid hormone

Thyroxine (T4) is the predominant form of thyroid hormone (TH). Hyperthyroidism, a condition associated with excess TH, is characterized by increases in metabolic rate, core body temperature and cardiac performance. In target tissues, T4 is enzymatically deiodinated to 3,5,3′-triiodothyronine (T3), a high-affinity ligand for the nuclear TH receptors TRα and TRβ, whose activation controls normal vertebrate development and physiology. T3-modulated transcription of target genes via activation of TRα and TRβ is a slow process, the effects of which manifest over hours and days. Although rapidly occurring effects of TH have been documented, the molecules that mediate these non-genomic effects remain obscure. Here we report the discovery of 3-iodothyronamine (T1AM), a naturally occurring derivative of TH that in vitro is a potent agonist of the G protein–coupled trace amine receptor TAR1. Administering T1AM in vivo induces profound hypothermia and bradycardia within minutes. T1AM treatment also rapidly reduces cardiac output in an ex vivo working heart preparation. These results suggest the existence of a new signaling pathway, stimulation of which leads to rapid physiological and behavioral consequences that are opposite those associated with excess TH.

Effect of Genetic Variation in the Organic Cation Transporter 1, OCT1, on Metformin Pharmacokinetics

The goal of this study was to determine the effects of genetic variation in the organic cation transporter 1, OCT1, on the pharmacokinetics of the antidiabetic drug, metformin. Twenty healthy volunteers with known OCT1 genotype agreed to participate in the study. Each subject received two oral doses of metformin followed by collection of blood and urine samples. OCT1 genotypes had a significant (P<0.05) effect on metformin pharmacokinetics, with a higher area under the plasma concentration–time curve (AUC), higher maximal plasma concentration (Cmax), and lower oral volume of distribution (V/F) in the individuals carrying a reduced function OCT1 allele (R61C, G401S, 420del, or G465R). The effect of OCT1 on metformin pharmacokinetics in mice was less than in humans possibly reflecting species differences in hepatic expression level of the transporter. Our studies suggest that OCT1 genotype is a determinant of metformin pharmacokinetics.

Predictability of the covalent binding of acidic drugs in man

Although metabolism via glucuronide conjugation has generally been considered a detoxification route for carboxylic acids, the newly discovered chemical reactivity of these conjugates, leading to covalent binding with proteins, is consistent with the toxicity observed for drugs containing the carboxylic acid moiety. Here we report that degradation rates (intramolecular rearrangement and hydrolysis) for 9 drug glucuronide metabolites show an excellent correlation (r2 = 0.995) with the extents of drug covalent binding to albumin in vitro. Furthermore, this binding capacity is predictable based on chemical structure of the acid and depends on the degree of substitution at the carbon alpha to the carboxylic acid. The in vivo covalent binding in humans for these drugs is also predictable (r2 = 0.873) when the extent of adduct formation is corrected for the measured plasma glucuronide concentrations. These results suggest that the structure of a carboxylic acid drug may predict the degree to which the corresponding acyl glucuronides will form covalent adducts that probably/possibly lead to toxicity. This information could be a useful adjunct in drug design.

Kidney Function and Age Are Both Predictors of Pharmacokinetics of Metformin

The effects of renal impairment and age on the pharmacokinetics of metformin were evaluated. The subjects, including 6 young, 12 elderly, and 3 middle‐age healthy adults and 15 adults with various degrees of chronic renal impairment (CRI) each were given a single, 850‐mg metformin HCl tablet. Multiple whole blood, plasma, and urine samples were collected and analyzed for metformin levels using a high‐performance liquid chromatography (HPLC) method. In healthy elderly individuals, the plasma and whole blood clearance/absolute bioavailability values [CL/F and (CL/F)b], and corresponding renal clearance values (CLR and CLR,b) of metformin were 35–40% lower than the respective values in healthy young individuals. These two groups did not differ significantly with respect to volume of distribution (Vd), time to peak concentration (tmax), and parameters related to metformins appearance in the urine. In the moderate and severe CRI groups, all clearance values were 74–78% lower than in the healthy young/middle‐age group, and all other evaluable pharmacokinetic parameters (with the exception of tmax) differed significantly in this group. In the mild CRI group, clearance values of metformin, which were 23–33% lower than in the young/middle‐age group, were the only parameters that differed significantly. Based on a regression analysis of the combined data, both creatinine clearance (CLcr*; corrected for body surface area) and age are predictors of metformin clearance, with the following model best fitting the data: CL/F [or (CL/F)b, CLR, CLR,b] = α + β·CLcr* + γ · CLcr*·age. Metformin should not be used in patients with moderate and severe CRI, or in patients with mild, but not absolutely stable, renal impairment. The initial and maximum doses in elderly patients and patients with stable mild CRI should be lowered to approximately one third that given to the general (i.e., patients without non‐insulin‐dependent diabetes) population.

Buprenorphine and naloxone co-administration in opiate-dependent patients stabilized on sublingual buprenorphine.

Buprenorphine and naloxone sublingual (s.l.) dose formulations may decrease parenteral buprenorphine abuse. We evaluated pharmacologic interactions between 8 mg s.l. buprenorphine combined with 0, 4, or 8 mg of naloxone in nine opiate-dependent volunteers stabilized on 8 mg s.l. buprenorphine for 7 days. Combined naloxone and buprenorphine did not diminish buprenorphines effects on opiate withdrawal nor alter buprenorphine bioavailability. Opiate addicts stabilized on buprenorphine showed no evidence of precipitated opiate withdrawal after s.l. buprenorphine-naloxone combinations. Buprenorphine and naloxone bioavailability was approximately 40 and 10%, respectively. Intravenous buprenorphine and naloxone produced subjective effects similar to those of s.l. buprenorphine and did not precipitate opiate withdrawal.

Pharmacokinetics and Pharmacodynamics of Metformin in Healthy Subjects and Patients with Noninsulin‐Dependent Diabetes Mellitus

This study was conducted to assess the effect of noninsulin‐dependent diabetes mellitus (NIDDM) and gender on the pharmacokinetics of metformin and to investigate whether or not metformin exhibits dose‐dependent pharmacokinetics. The pharmacodynamic effects (on plasma glucose and insulin) of metformin in patients with NIDDM and in healthy subjects also were assessed. Nine patients with NIDDM and 9 healthy subjects received 4 single‐blind single‐dose treatments of metformin HCl (850 mg, 1,700 mg, 2,550 mg, and placebo) and a multiple‐dose treatment of 850 mg metformin HCl (3 times daily for 19 doses). After each single‐dose treatment and the final dose of the multiple‐dose phase, multiple plasma and urine samples were collected for 48 hours and assayed for metformin levels. Plasma samples were also assayed for glucose and insulin levels. There were no significant differences in metformin kinetics in patients with NIDDM compared with healthy subjects, in men compared with women, or during multiple‐dose treatment versus single‐dose treatment. Plasma concentrations of metformin increase less than proportionally to dose, most likely due to a decrease in percent absorbed. In patients with NIDDM, single doses of 1,700‐mg or higher of metformin significantly decrease postprandial, but not preprandial, glucose concentrations and do not influence insulin concentrations. With multiple doses, both preprandial and postprandial glucose concentrations and preprandial insulin concentrations were significantly lower than with placebo. The effect of metformin on glucose level is correlated with the average fasting plasma glucose level without drug. In healthy subjects, single and multiple doses of metformin showed no effect on plasma glucose, but significantly attenuated the rise in immediate postprandial insulin levels.

Cytochrome P450 Eicosanoids are Activators of Peroxisome Proliferator-Activated Receptor α

Cytochrome P450 (P450) eicosanoids regulate vascular tone, renal tubular transport, cellular proliferation, and inflammation. Both the CYP4A ω-hydroxylases, which catalyze 20-hydroxyeicosatetraenoic acid (20-HETE) formation, and soluble epoxide hydrolase (sEH), which catalyzes epoxyeicosatrienoic acid (EET) degradation to the dihydroxyeicosatrienoic acids (DHETs), are induced upon activation of peroxisome proliferator-activated receptor α (PPARα) by fatty acids and fibrates. In contrast, the CYP2C epoxygenases, which are responsible for EET formation, are repressed after fibrate treatment. We show here that P450 eicosanoids can bind to and activate PPARα and result in the modulation of PPARα target gene expression. In transactivation assays, 14,15-DHET, 11,2-EET, and 20-HETE were potent activators of PPARα. Gel shift assays showed that EETs, DHETs, and 20-HETE induced PPARα-specific binding to its cognate response element. Expression of apolipoprotein A-I was decreased 70% by 20-HETE, whereas apolipoprotein A-II expression was increased up to 3-fold by 11,12-EET, 14,15-DHET, and 20-HETE. In addition, P450 eicosanoids induced CYP4A1, sEH, and CYP2C11 expression, suggesting that they can regulate their own levels. Given that P450 eicosanoids have multiple cardiovascular effects, pharmacological modulation of their formation and/or degradation may yield therapeutic benefits.

Functional analysis of polymorphisms in the organic anion transporter, SLC22A6 (OAT1)

Objectives The organic anion transporter, OAT1 (SLC22A6), plays a role in the renal elimination of many drugs and environmental toxins. The goal of this study was to identify and functionally characterize OAT1 variants as a first step towards understanding whether genetic variation in OAT1 may contribute to interindividual differences in renal elimination of xenobiotics. Methods As part of a larger study, 276 DNA samples from an ethnically diverse population were screened and 12 coding region variants of OAT1 were identified. The non-synonymous variants were then constructed and characterized in Xenopus laevis oocytes. A small family-based clinical study was conducted to determine the renal elimination of a model OAT1 substrate, adefovir (an antiviral agent) in human subjects who possessed a non-functional variant, OAT1-R454Q. Results Six non-synonymous variants were identified; two (OAT1-R50 H and OAT1-R293W) were present at ≥1% in at least one ethnic population. These two variants exhibited normal uptake of p-aminohippurate, ochratoxin A and methotrexate assayed in X. laevis oocytes. One variant, OAT1-R454Q, was non-functional with respect to the above substrates. In the clinical study, there was no significant decrease in the renal secretory clearance of adefovir in family members heterozygous for OAT1-454Q in comparison to those with the reference transporter, OAT1-454R. Conclusions These data indicate that the coding region of OAT1 has low genetic and functional diversity and suggest that coding region variants of OAT1 may not contribute substantially to interindividual differences in renal elimination of xenobiotics.

Combined Prostaglandin and Nitric Oxide Inhibition Produces Anatomic Remodeling and Closure of the Ductus Arteriosus in the Premature Newborn Baboon

After birth, the full-term ductus arteriosus actively constricts and undergoes extensive histologic changes that prevent subsequent reopening. These changes are thought to occur only if a region of intense hypoxia develops within the ductus wall after the initial active constriction. In preterm infants, indomethacin-induced constriction of the ductus is often transient and is followed by reopening. Prostaglandins and nitric oxide both play a role in inhibiting ductus closure in vitro. We hypothesized that combined inhibition of both prostaglandin and nitric oxide production (with indomethacin and N-nitro-l-arginine (L-NA), respectively) may be required to produce the degree of functional closure that is needed to cause intense hypoxia. We used preterm (0.67 gestation) newborn baboons that were mechanically ventilated for 6 d: 6 received indomethacin alone, 7 received indomethacin plus L-NA, and 16 received no treatment (control). Just before necropsy, only 25% of control ductus and 33% of indomethacin-treated ductus were closed on Doppler examination; in contrast, 100% of the indomethacin-plus-L-NA-treated ductus were closed. Control and indomethacin-treated baboons developed negligible-to-mild ductus hypoxia (EF5 technique). Similarly, there was minimal evidence of ductus remodeling. In contrast, indomethacin-plus-L-NA-treated baboons developed intense hypoxia in regions where the ductus was most constricted. The hypoxic muscle strongly expressed vascular endothelial growth factor, and proliferating luminal endothelial cells filled and occluded the lumen. In addition, cells in the most hypoxic regions were undergoing DNA fragmentation. In conclusion, preterm newborns are capable of remodeling their ductus, just like the full-term newborn, if they can reduce their luminal blood flow to a point that produces intense ductus wall hypoxia. Combined prostaglandin and nitric oxide inhibition may be necessary to produce permanent closure of the ductus and prevent reopening in preterm infants.

Pharmacokinetics and Subjective Effects of Sublingual Buprenorphine, Alone or in Combination with Naloxone Lack of Dose Proportionality

AbstractObjective: Buprenorphine and buprenorphine/naloxone combinations are effective pharmacotherapies for opioid dependence, but doses are considerably greater than analgesic doses. Because dose-related buprenorphine opioid agonist effects may plateau at higher doses, we evaluated the pharmacokinetics and pharmacodynamics of expected therapeutic doses. Design: The first experiment examined a range of sublingual buprenorphine solution doses with an ascending dose design (n = 12). The second experiment examined a range of doses of sublingual buprenorphine/naloxone tablets along with one dose of buprenorphine alone tablets with a balanced crossover design (n = 8). Participants: Twenty nondependent, opioid-experienced volunteers. Methods: Subjects in the solution experiment received sublingual buprenorphine solution in single ascending doses of 4, 8, 16 and 32mg. Subjects in the tablet experiment received sublingual tablets combining buprenorphine 4, 8 and 16mg with naloxone at a 4: 1 ratio or buprenorphine 16mg alone, given as single doses. Plasma buprenorphine, norbuprenorphine and naloxone concentrations and pharmacodynamic effects were measured for 48–72 hours after administration. Results: Buprenorphine concentrations increased with dose, but not proportionally. Dose-adjusted areas under the concentration-time curve for buprenorphine 32mg solution, buprenorphine 16mg tablet and buprenorphine/naloxone 16/4mg tablet were only 54 ± 16%, 70 ± 25% and 72 ± 17%, respectively, of that of the 4mg dose of sublingual solution or tablet. No differences were found between dose strengths for most subjective and physiological effects. Pupil constriction at 48 hours after administration of solution did, however, increase with dose. Subjects reported greater intoxication with the 32mg solution dose, even though acceptability of the 4mg dose was greatest. Naloxone did not change the bioavailability or effects of the buprenorphine 16mg tablet. Conclusion: Less than dose-proportional increases in plasma buprenorphine concentrations may contribute to the observed plateau for most pharmacodynamic effects as the dose is increased.