New lipid-lowering drugs: an update.

Lipid lowering is established as a proven intervention to reduce atherosclerosis and its complications. Statins form the basis of care but are not able to treat all aspects of dyslipidaemia. Many novel therapeutic compounds are being developed. These include additional therapeutics for low-density lipoprotein cholesterol, for example, thyroid mimetics (thyroid receptor beta-agonists), antisense oligonucleotides or microsomal transfer protein inhibitors (MTPI); triglycerides, for example, novel peroxosimal proliferator activating receptors agonists, MTPIs, diacylglycerol acyl transferase-1 inhibitors and high-density lipoprotein cholesterol (HDL-C), for example, mimetic peptides; HDL delipidation strategies and cholesterol ester transfer protein inhibitors and modulators of inflammation, for example, phospholipase inhibitors. Gene therapy for specific rare disorders, for example, lipoprotein lipase deficiency using alipogene tiparvovec is also in clinical trials. Lipid-lowering drugs are likely to prove a fast-developing area for novel treatments as possible synergies exist between new and established compounds for the treatment of atherosclerosis.

Pharmacokinetics and tolerability of GW420867X, a nonnucleoside reverse transcriptase inhibitor, following single escalating doses in healthy male volunteers.

The aim of the current study was to characterize the pharmacokinetics of GW420867X, a new nonnucleoside reverse transcriptase inhibitor, using a single escalating dose protocol in healthy volunteers. Four dose levels were investigated in sequential order: 300, 600, 900, and 1200 mg, with a ratio of 4:1 subjects receiving active or placebo treatment, respectively. Following single-dose administration, GW420867X was readily absorbed with a median time to peak concentration of 3 to 5 hours. GW420867X plasma exposure (AUC) was dose proportional but variable within the 300 to 1200 mg dose range. Less than dose-proportional increases were observed for Cmax. The terminal elimination t(1/2) was 50 hours, which supports once-daily dosing in future studies. Plasma trough concentrations of GW420867X at 24 hours after dosing were many fold greater than the in vitro IC50 HIV-1(HXB2) in MT4 cells. GW420867X was generally well tolerated following single-dose administration up to 900 mg; increased central nervous system-related adverse events were observed at higher doses. GW420867X had a favorable pharmacokinetic and safety profile that would enable this drug to be explored in future clinical studies with HIV-1 infected patients at doses that would provide appropriate safety and efficacy.

The current status of antihypertensive treatments: into the new millennium.

In this short review, we present a historical perspective of the treatment of hypertension, highlight some current issues and look to the possible future of antihypertensive therapy. The distribution of blood pressure within the population adopts a continuous, albeit somewhat skewed, distribution, so what constitutes hypertension? Conventionally, the disease has been defined as a level of blood pressure >> 140/90 mmHg. Accepting this 'arbitrary' definition infers that approximately one quarter of the adult population in the US are hypertensive [1]. This has significant implications in terms of the impact upon public health. We know that treatment of hypertension can prevent the serious consequences of cardiovascular disease: stroke, myocardial infarction (MI), heart failure and renal disease. Thus, it is important that raised blood pressure is both detected and effectively lowered. To what level should blood pressure be reduced. Conventionally, a level of 120/80 mmHg has been used to define normotension but there are indications that under certain circumstances this should not be the target. The question also arises as to whether it matters how blood pressure is treated. The choice of agent may ultimately depend upon the presence of any concomitant condition and risk factors. Recent trial evidence has concluded that therapy selected to treat raised blood pressure should take into account the overall cardiovascular risk profile of the patient [2].

Pharmacokinetics and safety of escalating single and repeat oral doses of GW420867X, a novel non-nucleoside reverse transcriptase inhibitor.

OBJECTIVE: To assess the pharmacokinetics, safety and tolerability of escalating oral doses of GW420867X, a non-nucleoside reverse transcriptase inhibitor, was investigated in healthy male volunteers in a randomized, double-blind placebo-controlled study. METHODS: Study subjects were divided into four groups of 12 subjects (10, 50, 100 and 200-mg dose groups) with eight subjects from each group receiving active treatment and the remaining four matched placebo. Subjects were initially administered a single dose of GW420867X or placebo, and following a 24- to 28-day washout period, re-exposed to the same treatment for 14 consecutive days. Safety measurements including clinical laboratory evaluations, ECG and vital signs were performed before, during and after dosing. RESULTS: Geometric mean GW420867X peak plasma concentrations (Cmax) following single oral doses of 10, 50, 100 and 200 mg were 160, 608, 1,000 and 1,662 ng/ml, respectively. Time to Cmax (tmax) increased from a median value of 1 h following the 10-mg dose, to 3 h after the 200-mg dose. Geometric mean plasma areas under the curves (AUC) were 4,325 (10 mg), 17,862 (50 mg), 35,295 (100 mg) and 62,338 ng/ml per hour (200 mg) and were proportionally less than the increase in the administered dose. Apparent terminal elimination half-life (t1/2) was approximately 50 h. Following repeat dosing, accumulation ratios based on plasma AUC were: 3.0+/-1.0 (10mg), 2.6+/-0.9 (50mg), 1.8+/-0.3 (100 mg) and 1.9+/-0.8 (200 mg) after 14 days of dosing compared to the corresponding single dose. In general, oral clearance (CL/F) was greater after 14 days and greater with higher doses except for the 10-mg dose group. Steady-state CL/F was 2.2, 3.4, 4.2, and 5.1 l/h for 10, 50, 100, and 200 mg, respectively. Steady-state was generally achieved within 7-10 days. Comparison of single and repeat dosing with GW420867X showed that Cmax increased by a factor of between 1.4 to 1.8, after 14 days of daily dosing to 288 (10 mg), 1,006 (50 mg), 1,401 (100 mg) and 2,613 (200 mg) ng/ml. These increases were proportionally less than the increase in the administered dose. GW420867X was well tolerated by subjects both after single and repeated dosing. Adverse effects reported by subjects on the active drug were similar to those receiving placebo. All episodes were rated as mild to moderate in severity and resolved spontaneously without further intervention. CONCLUSION: The pharmacokinetic findings of this study imply that systemic exposure to GW420867X decreases with increasing dose and displays time-variant pharmacokinetics, which suggests decreased absorption and/or increased clearance of GW420867X. The relatively long plasma half-life, of approximately 50 h, makes it suitable for once-daily dosing.

Relation between sodium-lithium countertransport and hypertriglyceridemia in type V hyperlipidemia.

Sodium-lithium countertransport (SLC) kinetics were measured in 30 patients with type V hyperlipidemia, 30 patients with type IIB hyperlipidemia on similar treatment, and 30 age- and sex-matched healthy controls. Clinical and laboratory data including basic anthropometry and blood pressure were obtained and blood was taken for detailed lipid biochemistry, glucose, insulin, and leptin measurements. Patients with type V hyperlipidemia were normotensive but more obese than controls, had elevated triglycerides, very low-density lipoprotein, glucose, and insulin; and reduced HDL cholesterol compared with type IIb controls. The median SLC activity (0.23 v 0.21 mmol Li+/L RBC/h) and median maximal velocity (0.33 v 0.31 mmol Li+/L RBC/h) were increased, but not significantly, compared to controls. In patients with type V hyperlipidemia SLC maximal velocity correlated with log triglycerides (r2 = 0.853; P < .001) and log very low-density lipoprotein (VLDL) triglycerides (r2 = 0.947; P < .001). Sodium-lithium countertransport maximal velocity correlated weakly with the homeostasis model assessment index of insulin resistance (r2 = 0.224; P = .06). The sodium affinity of the transporter did not differ between the groups and was independent of any of clinical or biochemical parameter studied. We conclude that VLDL triglyceride is strongly correlated with SLC maximal velocity and activity in patients with type V hyperlipidemia.

Effects of lipids in patients with familial hypercholesterolaemia on the kinetics of the sodium-lithium countertransporter.

Sodium-lithium countertransport kinetics were measured in 87 patients (50 male; 37 female) with heterozygous familial hypercholesterolaemia (FH) and a group of 38 age range and sex-distribution matched controls. Basic clinical data including basic anthropometry, blood pressure were obtained and blood was taken for detailed lipid biochemistry, glucose and insulin measurement. Patients with FH had elevated total cholesterol, low-density lipoprotein (LDL)-cholesterol and apolipoprotein B concentrations compared to controls. The activity and log transformed maximal velocity (Vmax) of the sodium-lithium countertransporter unlike the affinity (Km) were reduced in patients with FH compared to controls (geometric means 0.172 vs 0.217 mmol Li+/L.RBC.hr; P = 0.02; 0.237 vs. 0.317 mmol Li+/L.RBC.hr; P = 0.009 respectively). In multiple regression analysis, log normalised SLC activity correlated weakly with log triglyceride (beta = 0.225; P = 0.06) and cholesterol (beta = -0.112 P = 0.06). Log Vmax correlated with log triglyceride (beta = 0.307; P = 0.02), and high-density lipoprotein (HDL) (beta = 0.74; P = 0.03) whilst Km correlated with HDL (beta = 1.73; P<0.001) and apoAI (beta = -1.76; P = 0.0048), LDL (beta = -0.14; P = 0.05), and creatine kinase (beta = 0.003; P = 0.01). Cholesterol and triglyceride concentrations rather than insulin resistance seem to be the key features affecting the environmental alteration of sodium lithium countertransporter Vmax in patients with familial hypercholesterolaemia.

Transport of phytanic acid on lipoproteins in Refsum disease.

Patients with Refsum disease accumulate significant quantities of phytanic acid in adipose and neural tissue. The accumulation can be reversed by following a diet low in phytanic acid, yet the mechanism of transport of this fatty acid is obscure. We investigated the distribution of phytanic acid in different lipoprotein subfractions in 11 patients with Refsum disease and 9 unaffected siblings. Plasma phytanic acid was distributed on VLDL (16.2% +/- 12.2%), IDL (1.77% +/- 1.64%), LDL (34.8% +/- 12.6%) and HDL (14.3% +/- 7.87%). No correlations with any parameter were seen with total phytanic acid content. Weak nonsignificant correlations were found with the fractional distribution of phytanic acid and VLDL triglyceride (r = 0.35; p = 0.12) and plasma HDL-cholesterol (r = 0.32; p = 0.16) and with LDL:HDL cholesterol ratio (r = 0.33; p = 0.14). Significant correlation of the fractional distribution of phytanic acid on lipoprotein particles was noted with the ratio of apolipoprotein B: apolipoprotein A1-containing particles (r = 0.46; p = 0.03) and apolipoprotein B: apolipoprotein A1 in HDL2 (r = 0.53; p = 0.01). This suggests that the import-export balance for phytanic acid in plasma is related to forward and reverse cholesterol transport on lipoprotein particles, and only weakly to plasma cholesterol and triglycerides. These ratios of apolipoprotein particles may play a significant role in determining the rate of phytanic acid elimination in patients with Refsum disease.

Influence of plasma phytanic acid levels in Refsum's disease on the behaviour of the erythrocyte membrane sodium-lithium countertransporter.

BACKGROUND: Abnormal behaviour of the erythrocyte membrane sodium-lithium countertransporter (SLC) is associated with plasma triglyceride concentrations. Refsum's disease is characterized by progressive neurological dysfunction and accumulation of phytanic acid, an isoprenoid fatty acid, in fat-containing tissues. METHODS: This study explored the effects of plasma phytanic acid on SLC kinetics in nine Caucasian patients with Refsum's disease and in age- and sex-matched Caucasian control subjects. RESULTS: A dose-dependent association was seen between countertransporter maximal velocity and phytanic acid content of low-density lipoprotein (LDL)-cholesterol (r = -0.61, r = -0.65 respectively; P = 0.05, P = 0.04) and high-density lipoprotein (HDL)-cholesterol (r = -0.81, -0.82 respectively; P = 0.005, P = 0.003). No significant association was seen with the sodium affinity of the transporter (r = -0.44, P = 0.20, for LDL; and -0.43, P = 0.21, for high-density lipoprotein). CONCLUSION: These findings suggest that phytanic acid may alter the behaviour of the sodium-lithium countertransporter.

Kinetic characteristics of the erythrocyte sodium-lithium countertransporter in black normotensive subjects compared with three other ethnic groups.

Sodium-lithium countertransport activity has been proposed as a marker for hypertension and is lower in black compared to Caucasian subjects both with and without vascular disease. The question arises of what is the primary kinetic locus of the altered behaviour of the countertransporter in black subjects and whether this is also seen in normotensive subjects from other non-Caucasian ethnic groups. We studied the sodium-lithium countertransporter in four ethnic groups (black [n = 45], Caucasian [n = 45], Chinese [n = 40], and South Asian [n = 39]) of age-matched normotensive males (age range 18-35 y) with no first-degree family history of hypertension or diabetes. The clinical and laboratory characteristics of the subjects were similar in all four ethnic groups. Minor differences noted were: significantly higher mean height, weight and serum creatinine concentration (P < 0.001) and significantly lower plasma triglyceride concentration (P = 0.02) in the black compared to the other study groups. Sodium-lithium countertransport activity (mmol Li/l RBC h) was significantly lower in the black subjects (0.113 [0.013-0.265]) compared with the other groups (Caucasian, 0.247 [0.037-0.614]; Chinese, 0.210 [0.100-0.707]; South Asian, 0.211 [0.037-0.617]; P < 0.001). No differences were noted between the four study groups in respect of kNa. Mean (s.d.) Vmax values (mmol Li/l RBC h) were also reduced in the black subjects (0.152 [0.088]) compared to the other ethnic groups (Caucasian, 0.376 [0.159]; Chinese, 0.364 [0.182]; South Asian, 0.329 [0.155]; P < 0.001) and there was a strong relationship between countertransport activity and Vmax (r > 0.52; P < 0.001; for each of the study groups). The differences in mean Vmax noted between the Caucasian, South Asian and Chinese subjects were not significant. These results show that, when compared with three other selected ethnic groups, black subjects demonstrate an altered behaviour with respect to Vmax of the sodium-lithium countertransporter, which occurs in the absence of demonstrable vascular pathology.

Characterization of the erythrocyte sodium-lithium countertransporter: limitations and assumptions of traditional and kinetic methodologies.

The present work examined the key elements featuring in the various methods used to characterize the erythrocyte sodium-lithium countertransport. Effects of medium composition on lithium efflux were investigated in 20 subjects. Mean lithium efflux (mmol Li/l RBC.h) into a 150 mM sodium medium was significantly higher than efflux into a revised sodium-rich medium (149 mM) containing 1 mM Mg (0.335 +/- 0.100 vs. 0.298 +/- 0.085 respectively; P < 0.03). Mean lithium efflux into sodium-free media where sodium had been entirely replaced by magnesium, was significantly lower than efflux into a choline-based medium containing only 1 mM magnesium (0.088 +/- 0.027 vs. 0.109 +/- 0.034 respectively; P = 0.03). Sodium-lithium countertransport activity and the transporter's kinetic profile were measured simultaneously in 35 subjects using traditional choline-based and kinetic methodologies. There was a significant correlation between countertransport activity and maximal rate of turnover (Vmax) (r = 0.62; P < 0.001); Vmax values were consistently greater than their corresponding countertransport activities (P < 0.001). On subdividing the subject group into tertiles based on the Michaelis-Menten constant (km) values (mM), < 75, 75-150 and > 150, the slopes of the regression lines for each group diminished progressively (0.64, 0.49 and 0.23 respectively), correlations within each group remained significant (P < 0.001, P < 0.001 and P < 0.02). No significant correlation was found between km values and countertransport activity (r = 0.035; P = NS). Increasing the number of points representing sodium concentrations within the range 0-150 mM, improved the confidence in the emerging estimates of Vmax and km obtained by linear transformation. Comparison of kinetic data derived using four different analytical methods (two linear transformations, a nonlinear regression and a statistical method), showed no significant differences between the estimates yielded for either Vmax (P = 0.88. NS) or km (P = 0.92, NS). This study has highlighted the critical roles of assay conditions and derivation techniques used when measuring sodium-lithium countertransport, emphasizing the need for standardization of the methodology.

Kinetic behavior of the erythrocyte sodium-lithium countertransporter in nonnephropathic diabetic twins.

Elevated erythrocyte sodium-lithium countertransport activity occurs in diabetes and may be genetically mediated. The relation of this abnormality to the disease and its complications is unclear. To remove confounding genetic factors and the impact of complications, we studied sodium-lithium countertransport activity together with its kinetic components, maximal rate of turnover (Vmax) and external affinity for sodium (kNa), in identical-twin pairs discordant for insulin-dependent diabetes who were normotensive and had no evidence of nephropathy. Fifteen twin pairs were studied along with the same number of healthy control subjects matched with the twins for gender, age, and body mass index. Clinical and laboratory characteristics of the twins and controls were similar, with the exception that whole blood glucose and glycated hemoglobin concentrations were higher in diabetic twins (P < .001). Comparison of countertransport activity between nondiabetic and diabetic twin groups failed to uncover any significant differences (P = .30, Wilcoxon). Similarly, there were no differences in countertransport activity between the nondiabetic twin group and the controls (P = .38, Mann-Whitney). Furthermore, no associations were noted between residual activity values and residual data of any of the other clinical or laboratory characteristics measured. Comparison of Vmax between nondiabetic and diabetic twin groups showed a significant elevation in the diabetic twins (0.515 + 0.220 v 0.439 + 0.229 mmol Li/L RBC x h, P = .049, paired t test). By contrast, no significant differences were noted between the nondiabetic twin group and the controls (P = .15, unpaired t test). Comparison of kNa between nondiabetic and diabetic twin groups found no significant differences in kNa (P = .42, Wilcoxon). Similarly, there were no differences in kNa between nondiabetic twins and controls (P = .14, Mann-Whitney). Neither the residual data for Vmax nor kNa showed any association with the residual data of any of the other clinical or laboratory characteristics measured. When intertwin correlations were examined, all three parameters describing the behavior of the sodium-lithium countertransporter showed significant intertwin correlations (activity, r = .51, P = .04; Vmax, r = .82, P = .001; kNa, r = .76, P = .001). In conclusion, the diabetic state has a small effect on the Vmax of the sodium-lithium countertransporter. Failure to consider the complex nature of the activity measurement is likely to have been partly responsible for earlier confusion with regard to the effect of diabetes on the countertransporter, since experimental conditions varied between studies and individual kinetic components were not measured. The associations between twins in this study with respect to Vmax and kNa indicate a genetic influence on both constants of the countertransporter. Vmax appears also to be sensitive to certain as yet unidentified environmental factors.

Controversies surrounding erythrocyte sodium-lithium countertransport.

BACKGROUND: Interest originally arose in ouabain-insensitive lithium transport across erythrocyte membranes when it was found that lithium could substitute for sodium, either undergoing 1:1 lithium exchange or 1:1 sodium-lithium countertransport in a manner that follows Michaelis-Menten kinetics. Elevation of the sodium-lithium countertransport activity in hypertension was first noted in 1980 and found to be a genetically linked phenomenon. This observation has since been confirmed on several occasions and associations with other diseases such as diabetes have been noted. Nevertheless, many unanswered questions remain about the clinical significance of disturbed sodium-lithium countertransport and its pathological basis. METHODS: Traditional methods for characterizing the sodium-lithium countertransporter have depended on determining differences between lithium fluxes into sodium-rich and sodium-free media. There have been inherent problems in deciding on suitable sodium substitutes. Of the available alternatives, choline has emerged as having advantages over magnesium. Reports in the literature have often failed to take into account varied assay conditions, making comparisons of data from different laboratories difficult. A further complexity has been the realization that sodium-lithium countertransport activity incorporates two key elements in the form of Vmax and k(m). Kinetic studies have shown independent variation in these two parameters with various disease states. RESULTS: Much of the published work to date has continued to rely on measurement of countertransport activity, with magnesium acting as the predominant sodium-substitute. This has occurred despite the undoubted benefits obtained from kinetic analysis. Where kinetics of the sodium-lithium countertransporter have been determined, there have emerged clear associations between Vmax and environmental influences such as plasma lipids with elevated values in dyslipidaemic states including diabetes. The affinity constant, k(m), is more clearly under genetic control and has independent associations with vascular disease. CONCLUSION: Study of the erythrocyte sodium-lithium countertransporter has revealed interesting relationships between altered behaviour of the transporter and specific disease states. Although still somewhat of an enigma, this transporter is emerging as an important membrane constituent whose further study may help us to understand the molecular mechanisms leading to vascular disease.

Angiotensin-converting enzyme (ACE) gene polymorphism and the erythrocyte sodium-lithium countertransporter (SLC) phenotype in hypertension.

It has been suggested that the association between altered behaviour of the erythrocyte sodium-lithium countertransporter (SLC) and hypertension could be secondary to its association with the risk of vascular disease rather than raised blood pressure (BP) per se. Homozygosity for the angiotensin-converting enzyme (ACE) deletion allele has also been linked to a more severe phenotype for cardiovascular disease. The present study investigated whether there is an association between these two indicators of vascular risk in patients with hypertension. The kinetic characteristics of the countertransporter (SLC activity, Vmax and KNa) of patients having the ID ACE-genotype (n = 16) were compared with those patients who had the DD genotype (n = 12). The median (range) SLC activity (mmol Li/l Red Blood Cells.h) in the ID (0.221 [0.061-0.422]) and DD (0.173 [0.094-0.408]) groups were similar (P = 0.28; Mann-Whitney). No significant differences in Vmax (mmol Li/l RBC.h) emerged between the two study groups (0.279 + 0.124 [ID] vs 0.244 + 0.123 [DD]; P = 0.46; unpaired Student's t-test); similarly, no differences emerged between the two groups with respect to KNa (median [range]; ID, 39.8 [12.4-84.4] vs DD, 35.9 [14.6-78.3]; P = 0.47). These data suggest that the SLC phenotype and the ACE-D allele dose are risk factors for cardiovascular disease that function independently of one another.

Kinetic characteristics of the erythrocyte sodium-lithium countertransporter in subjects with coronary artery disease.

Sodium-lithium countertransport activity, external affinity for sodium (kNa) and maximal rate of turnover (Vmax), were characterized in 21 male subjects (aged 45 to 65 years) with angiographically proven coronary artery disease; these were compared with a matched group of healthy controls. No significant differences in countertransport activity were noted between the coronary artery disease patients and the healthy controls. By contrast, the median [range] kNa in the coronary artery disease group (8.5 [2.6 to 30.5] mmol/L Na) was significantly lower than that in the controls (59.9 [5.9 to 240.5] mmol/L Na; P < .0001). This reduction was accompanied by a significantly lower mean Vmax (controls 0.403 +/- 0.187 v coronary artery disease group 0.248 +/- 0.121 mmol Li/L RBC/h; P < .01). The findings suggest that disturbed behavior of the sodium-lithium countertransporter is not confined to hypertension but may represent a broader-based membrane dysfunction associated with vascular disease.