The effects of probucol on plasma lipoproteins in polygenic and familial hypercholesterolaemia

Effect of probucol on cytochrome P450 activities in human liver microsomes

The effects of probucol, a cholesterol-lowering agent, on several cytochrome P450 (CYP) isoform-specific reactions in human liver microsomes were investigated to predict drug interactions with probucol in vivo from in vitro data. The following eight CYP catalytic reactions were used in this study: CYP1A1/2-mediated 7-ethoxyresorufin O-deethylation, CYP2A6-mediated coumarin 7-hydroxylation, CYP2B6-mediated 7-benzyloxyresorufin O-debenzylation, CYP2C8/9-mediated tolbutamide methylhydroxylation, CYP2C19-mediated S-mephenytoin 4'-hydroxylation, CYP2D6-mediated bufuralol 1'-hydroxylation, CYP2E1-mediated chlorzoxazone 6-hydroxylation, and CYP3A4-mediated testosterone 6beta-hydroxylation. Probucol had neither stimulatory nor inhibitory effects on CYP1Al/2, 2A6, 2B6, 2C8/9, 2C19, 2D6, 2E1, and 3A4 activities at concentrations up to 300 microM, indicating that probucol, at the expected therapeutic concentrations, would not be predicted to cause clinically significant interactions with other CYP-metabolized drugs.

Effects of probucol on phase transition and fluidity of phosphatidylcholine membranes: a spin label study

Spin labeling methods were applied to study the structure and dynamics of phosphatidylcholine membranes as a function of temperature and the mole fraction of probucol. Multilamellar liposomes made of dimyristoylphosphatidyclcholine, dipalmitoylphosphatidylcholine both saturated, and egg yolk phosphatidylcholine, an unsaturated membrane, were used. In fluid phase membranes probucol was found to increase the order and decrease the motional freedom of alkyl chains of lipids as shown with stearic acid spin labels. The effect of probucol on order and motional freedom is more pronounced in the membrane center (16-doxylstearic acid spin label position) than in the near polar headgroup region (5-doxylstearic acid spin label position). The presence of unsaturation in alkyl chains significantly decreased the ordering effect of probucol. The main phase transition temperature of saturated bilayers was lowered by 2 degrees C in the presence of 3 mol% of probucol and significantly broadened at higher concentrations as measured with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) partitioning. Also, pretransition was no longer observed in the presence of probucol. In gel phase membranes, the effect of probucol was complex. Close to the main phase transition the motion of alkyl chains was increased, showing a regulatory effect of probucol on membrane fluidity. It is proposed that probucol is located in the membrane center as opposed to vitamin E, which locates its phenolic -OH group at the membrane surface; therefore, it inhibits lipid peroxidation in this region which is less accessible to vitamin E.

Probucol protects low-density lipoproteins from in vitro and in vivo oxidation

Twelve patients with primary hypercholesterolemia were treated for 12 weeks with probucol (500 mg b.i.d.). For each patient low density lipoproteins (LDL), isolated by ultracentrifugation were subfractionated by ion exchange high resolution chromatography in order to evaluate the content of a more electronegatively charged LDL (LDL-), a small subfraction that probably represent a circulating oxidatively modified lipoprotein. The treatment induced a 17% reduction of total LDL and 43% reduction of LDL-. By thin layer chromatography the probucol content in LDL- was a quarter of that in normally charged LDL. Under basal conditions, native LDL incubated for 24 h with 3 microM copper sulphate shows a net increase in electrophoretic mobility, an increase in relative fluorescence intensity and a reduction in vitamin E content, thus indicating peroxidative damage. After treatment with probucol, no significant changes of electrophoretic mobility, fluorescence and vitamin E content are detectable. LDL isolated from patients treated with probucol thus become resistant to oxidation by copper ions. The observed reduction of LDL- after treatment with probucol, confirms in vivo the antioxidant role of the drug and support the hypothesis that circulating LDL- may be linked to an oxidative process occurring in vivo.

Mechanisms of high-density lipoprotein reduction after probucol treatment: changes in plasma cholesterol esterification/transfer and lipase activities

Probucol treatment results in a significant reduction of plasma high-density lipoprotein (HDL) levels. Since the remodeling of HDL within the plasma compartment is a crucial determinant of HDL levels, the activities of several factors participating in the process, ie, lecithin:cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP), and lipoprotein and hepatic lipases (LPL, HL), were evaluated in 15 hypercholesterolemic patients treated with probucol (1 g/d) for 8 weeks. Drug treatment was associated with significant reductions of HDL cholesterol ([HDL-C] -32%), HDL2-C (-65%), HDL3-C (-22%), apolipoprotein (apo)A-I (-27%), and apo A-II (-11%) levels and with the accumulation of small HDL in plasma. CETP activity increased by 48%, with minor changes in LCAT (-7%), LPL (+4%), and HL (-7%) activities. By linear regression analysis, CETP activity correlated inversely with HDL-C, HDL2-C, and apo A-I levels (r = -.63, -.52, and -.73, respectively) and with HDL particle size. In multivariate analysis, CETP activity was the strongest predictor of HDL-C levels, apo A-I levels, and HDL particle size. The hypothetical mechanism of probucol is a stimulation of CETP activity, resulting in the formation of triglyceride (TG)-enriched HDL. These are acted on by HL, leading to the accumulation of small HDL in plasma.

The effects of probucol on lipoprotein metabolism in the rat

The effects of probucol on liver and intestinal apolipoprotein, LDL-receptor and hepatic lipase gene expression, as well as plasma lipid and apolipoprotein levels and liver lipase activity were evaluated in male rats. Administration of probucol decreased plasma triacylglycerols, without affecting plasma cholesterol. Plasma apo E and apo B concentrations increased after probucol. Since liver and intestinal apo B and apo E mRNA levels remained unchanged, this increase could be attributed to a delayed clearance by the LDL-receptor, whose mRNA levels dropped by 50% in the liver. For the HDL-apolipoproteins, only liver apo A-IV mRNA levels decreased after probucol, which was reflected by a fall of plasma apo A-IV. Neither hepatic lipase activity nor mRNA levels were significantly influenced by probucol.

Probucol increases cholesteryl ester transfer protein activity in hypercholesterolaemic patients

Probucol, a widely used lipid lowering drug, reduces both low- and high-density (LDL and HDL) lipoprotein levels and can induce a regression of tissue lipid deposits in both animals and man. The suggested mechanism(s) involve the prevention of LDL oxidative modifications and, possibly, an improvement in the reverse cholesteryl ester transport system. Probucol administration to 10 hypercholesterolaemic patients increased the activity of the cholesteryl ester transfer protein (CETP) by 50%. The rise of CETP activity was significantly related with the plasma steady-state drug levels (r = 0.51, P less than 0.005), thus suggesting that probucol may directly stimulate CEPT synthesis and/or release. Furthermore, CETP activity was inversely related with HDL-cholesterol levels, both in the whole series of 10 patients (r = -0.56, P less than 0.001) and, more so, in the single individuals (r between -0.77 and -0.97), thus suggesting that the reduction of plasma HDL-cholesterol levels is a direct consequence of CETP stimulation. These findings support the hypothesis that an improvement in the reverse cholesteryl ester transport is a major mechanism of probucol and that this may explain the drug induced plasma lipoprotein changes.

Effects of probucol on plasma lipoprotein subfractions and activities of lipoprotein lipase and hepatic triglyceride lipase

The effects of 12 weeks treatment with probucol on plasma lipoprotein subfraction levels and on LPL and HTGL activities were investigated. Plasma VLDL-C, VLDL-TG, VLDL-apo B levels were not changed. Probucol significantly reduced plasma IDL-C and IDL-apo B levels by 26.7% and 23.8%, respectively. Plasma cholesterol and apo B levels of large light LDL (LDL1) were decreased significantly by 27.8% and 23.2% by probucol treatment. Plasma cholesterol and apo B levels of small heavy LDL (LDL2) remained unchanged. Probucol markedly reduced plasma HDL2 levels. The reduction rates of plasma TC, TG and apo A-I levels of HDL2 were 43.0%, 43.6% and 47.0%. Probucol significantly decreased HDL3-C and HDL3-apo A-I levels by 18.0% and 19.2%. LPL activities in the post-heparin plasma were decreased significantly from 2.53 +/- 0.71 mumol free fatty acids (FFA)/ml/h to 1.71 +/- 0.71 mumol FFA/ml/h by probucol while HTGL activities remained unchanged. We conclude that probucol suppresses LPL activity and decreases plasma IDL, LDL1 and HDL2 levels due to disturbances of VLDL conversion to LDL1 via IDL and of HDL3 conversion to HDL2.

Increase in plasma cholesteryl ester transfer protein during probucol treatment. Relation to changes in high density lipoprotein composition

Probucol is a hypolipidemic agent that causes a marked decrease in high density lipoprotein (HDL) cholesterol. To investigate the mechanism of this effect, two studies were performed in hypercholesterolemic patients who had been stabilized previously on diet and were not receiving other lipid-lowering medication. Plasma cholesteryl ester transfer protein (CETP) concentrations were measured in fasting plasma samples before and after 10 weeks of probucol therapy using a sensitive and specific radioimmunoassay. Plasma total and low density lipoprotein cholesterol concentrations decreased, whereas apolipoprotein (apo) B was unchanged. Plasma apo E concentrations increased markedly. HDL cholesterol and apo A-I decreased in all subjects. These effects of probucol were accompanied by even more striking changes in plasma CETP concentrations, which increased by a mean of 64%. In a second study of six hypercholesterolemic subjects, the time-course effects of probucol on CETP and HDL subspecies were studied. Significant increases in plasma apo E and in CETP occurred after 4 weeks, and CETP, but not apo E, increased further after 16 weeks of treatment. Concomitant and opposite changes occurred in HDL composition, with decreases in HDL cholesterol and lipoprotein containing apo A-I. The increase in plasma CETP concentrations, the decrease in HDL cholesterol, and the increase in plasma apo E concentrations observed during probucol treatment are changes consistent with a postulated increase in reverse cholesterol transport via the remnant pathway.

Antioxidant activity of probucol and its effects on phase transitions in phosphatidylcholine liposomes

The effect of probucol on the phase behavior of dimyristoylphosphatidylcholine (DMPC) was examined by fluorescence polarization and differential scanning calorimetry (DSC). Probucol broadens and shifts the temperature of the main phase transition of DMPC liposomes as measured by fluorescence polarization with diphenylhexatriene and trimethyl-ammonium-diphenylhexatrine at concentrations as low as 5 mole%. As measured by DSC, probucol reduces the transition temperature of the gel----liquid-crystalline phase transition of DMPC by approx. 2 C degrees at all concentrations above about 5 mole% probucol and eliminates the pretransition at less than 1 mole%. In addition, the phase transition of DMPC is broadened and the enthalpy of the transition reduced by approx. 50%. Even at high concentrations of probucol, the gel----liquid-crystalline phase transition of DMPC is not eliminated. Similar effects are observed with dipalmitoylphosphatidylcholine liposomes. Based on these DSC measurements, measurements of the melting of probucol in dry mixtures with DMPC and observations of probucol mixtures with DMPC under polarizing optics, the maximum solubility of probucol in DMPC is approx. 10 mole%. This concentration exceeds that required (approx. 0.5 mole%) to prevent peroxidation of 10 mole% arachidonic acid in DMPC liposomes for 30 min in the presence of 0.05 mM Fe(NH4)(SO4)2 at 4 degrees C. Thus, probucol has a limited solubility in saturated phosphatidylcholine bilayers, but is an effective antioxidant at concentrations lower than its maximum solubility.

Mechanisms of HDL reduction after probucol. Changes in HDL subfractions and increased reverse cholesteryl ester transfer

Treatment with probucol, a widely used lipid-lowering agent, is associated with a significant reduction of high density lipoprotein (HDL) cholesterol levels, but with an apparently improved removal of cholesteryl esters from tissues (e.g., from tendon xanthomas). The effects of probucol (500 mg twice daily) on HDL subfraction distribution and cholesteryl ester transfer activity were tested in 12 patients with stable type II hyperlipidemia [low density lipoprotein (LDL) cholesterol greater than 180 mg/dl] after a placebo-controlled cross-over trial. Probucol significantly lowered total cholesterol (-13.8%), LDL cholesterol (-9.1%), and HDL cholesterol (-30%). By rate zonal ultracentrifugation, a marked reduction of HDL2 cholesterol (-68%) was shown, whereas changes in HDL3 were less significant (-21%). These findings were confirmed by polyacrylamide gradient gel electrophoresis, typically showing a reduction or disappearance of HDL2b particles and the prevalence of particles in the HDL3a range. Cholesteryl ester transfer from HDL to lower density lipoproteins was significantly increased (30%) in all patients. These findings suggest that, in addition to the well-documented in vitro changes (prevention of LDL peroxidation and macrophage uptake), probucol characteristically modifies HDL particle distribution in vivo, and is associated with a significant increase of cholesteryl ester transfer activity.

Lack of effect of probucol on atheroma formation in cholesterol-fed rabbits kept at comparable plasma cholesterol levels

Rabbits were fed cholesterol for 14 weeks to study the effect of probucol on atheroma formation. Three groups of animals were investigated: group CHOL was fed 1% cholesterol and served as control for group P + CHOL. fed 1% cholesterol and 1% probucol from the onset till the end of the experiment: group CHOL + P received 1% cholesterol throughout the experiment and 1% probucol during the last 4 weeks only. Plasma cholesterol concentrations were monitored at frequent intervals and were modulated by dietary perturbations so that the areas under the curve expressing plasma cholesterol changes with time, were similar in probucol and non-treated rabbits. The efficacy of long-term probucol treatment was evidenced by a significant reduction in plasma apolipoprotein A-I throughout the experiment and lower plasma TBARs during the first 6 weeks, when the hypocholesterolemic effect of probucol was also seen. Two weeks prior to the termination of the experiment, the rabbits were injected with rabbit plasma labeled with [3H]cholesteryl linoleyl ether [( 3H]CLE). Aortic atheromatosis was quantified by determination of total and cholesteryl ester (CE). The aortic cholesterol content was related to the arch, thoracic and abdominal segments, to the surface area of each segment or its dry defatted weight. Total and esterified cholesterol were highest in the aortic arch in all 3 groups when related to any of the above mentioned parameters. No statistically significant difference in aortic total cholesterol and CE content was seen among the three groups studied. The [3H]CLE recovered in the aortic segment correlated with the CE content and the [3H]CLE (dpm)/mg CE in all segments was similar. No statistically significant difference in the [3H]CLE recovered in the aortic segments among the 3 groups was seen. We conclude that in cholesterol-fed rabbits, in which the plasma cholesterol levels were maintained at comparable levels, probucol treatment did not affect plasma CE influx into the aorta and did not attenuate development of aortic atherosclerosis.

Probucol, high-density lipoprotein metabolism and reverse cholesterol transport

Accumulation of cholesterol within the arterial wall reflects an imbalance between delivery and efflux. Monocyte-derived macrophages play a major role in arterial wall cholesterol accumulation. Using tracer methodology in a rabbit model, several investigators have estimated the rate of cholesterol delivery and thus the steady-state rate of efflux to be between 0.4 and 2.4 micrograms/cm2/hour. The process responsible for arterial wall cholesterol efflux, "reverse cholesterol transport," can be conceptualized as a sequence of events including (1) loss of cell cholesterol, (2) intravascular cholesterol transport, (3) hepatic cholesterol uptake, and (4) biliary secretion. Work by many investigators has characterized these individual processes.

Increased levels of messenger ribonucleic acid for apolipoprotein E in the spleen of probucol-treated rabbits

To study the organ-specific effect of probucol, a potent cholesterol-lowering drug, on apolipoprotein(apo)E synthesis, rabbit apoE complementary deoxyribonucleic acid (cDNA) clones were isolated and apoE messenger ribonucleic acid (mRNA) levels were determined in various tissues isolated from probucol-treated rabbits. A 0.54 kb apoE cDNA was cloned from a rabbit liver cDNA library using a synthetic oligonucleotide probe. The amino acid sequence deduced from an apoE coding region indicated 78% homology for human apoE and 63% homology for rat apoE. RNA blot analysis showed that apoE mRNA was most abundant in the liver, then in the brain and spleen. The relative amounts of apoE mRNA were determined in tissues of rabbits fed a normal diet, or high-cholesterol (1%) diet with or without probucol (1%). ApoE mRNA levels increased 2- to 4-fold in the spleen and brain after cholesterol feeding for 4 weeks and were higher by 52 to 70% in the spleen of probucol-treated rabbits than in nontreated rabbits. This induction of apoE mRNA occurs within 7 days after the initiation of probucol treatment. However, apoE mRNA levels in the liver, a major apoE-synthesizing organ, were not enhanced after probucol treatment. These results indicate that probucol induces apoE mRNA expression specifically in the spleen and may affect apoE-mediated lipoprotein metabolism, the so-called reverse cholesterol transport.

Influence of probucol administration on lipoprotein cholesterol and apolipoproteins in normolipemic males

In order to interpret the known lipoprotein changes in probucol-treated patients, serum concentrations of apolipoproteins (A-I, A-II, B, C-II, C-III, E) were measured before, during and after probucol administration (2 X 500 mg p.d.), in 16 healthy males (30.3 +/- 5.6 years old). Cholesterol concentrations were determined in LDL and VLDL fractions as well as in HDL subfractions which were isolated by preparative ultracentrifugation. In addition, apolipoprotein A-I and A-II concentrations were measured in the HDL subfractions. Compared with the baseline values, significant apolipoprotein changes were found in the serum apolipoprotein A-I (151 +/- 18 to 115 +/- 31 mg/dl; P less than 0.001) and C-II levels during administration. The HDL subfraction analysis showed that the decrease of HDL-cholesterol and apolipoprotein A-I (59.9 +/- 23.5 to 34.4 +/- 16.4 mg/dl, P less than 0.001, and 65.7 +/- 49.0 to 37.5 +/- 23.5 mg/dl, P less than 0.05, respectively) was predominantly related to the HDL2b subfraction (d = 1.063-1.100 g/ml).

Probucol reduces the rate of association of apolipoprotein C-III with dimyristoylphosphatidylcholine

The effect of low concentrations of probucol and cholesterol on the association of dimyristoylphosphatidylcholine with human plasma apolipoprotein C-III was studied. Liposomes of dimyristoylphosphatidylcholine with or without probucol or cholesterol were prepared by swelling the lipids in buffer at 37 degrees C. The association of apolipoprotein C-III with the liposomes was determined at 24 degrees C by measuring the rate of clearing of turbidity at 400 nm following addition of protein. At a weight ratio of probucol/dimyristoylphosphatidylcholine of 1:25 (5 mol% probucol), the rate of clearing of liposomes was decreased by 60%; 5 mol% cholesterol had no effect on the clearing rate. Liposomes were then added to the preformed apolipoprotein C-III/lipid micelles. In the absence of probucol, the added liposomes cleared rapidly regardless of the presence or absence of cholesterol. With 5 mol% probucol, almost no decrease in absorbance was noted on addition of liposomes to the micelles. These data show that probucol reduces the rate of association of an apolipoprotein with lipid and suggests that the interaction of probucol with lipid may modify the assembly and/or metabolism of lipoproteins.

Apolipoprotein E polymorphism and plasma cholesterol response to probucol

Probucol has been shown to be an effective and well-tolerated cholesterol-lowering drug. However, response in terms of cholesterol reduction has been shown to vary significantly among individuals. The purpose of this study was to assess the role of apolipoprotein E polymorphism in determining this variation. A retrospective study of 89 hypercholesterolemic type II patients who had been treated with probucol (1 g/d) and for whom the apolipoprotein E phenotype was known was carried out. The patients were first grouped into those with heterozygous familial hypercholesterolemia (FH) and those considered to have other forms of hypercholesterolemia (non-FH). Further subclassification of the individuals in both groups as IIa or IIb, allowed the definition of four diagnostic classes, FH IIa or IIb and non-FH IIa or IIb. Among these classes there was no significant heterogeneity for the relationship between response and age or sex. After correction for between-class heterogeneity in duration of probucol treatment, comparison of individuals with the apo E3/3 phenotype with those carrying the epsilon 4 allele showed significant differences in cholesterol reduction both absolute change and percent change. Further contrasts between diagnostic and apo E genotype stratifications of these data showed that the FH patients carrying the epsilon 4 allele had the greatest reduction in cholesterol level.

Effects of probucol on the composition and in vitro catabolism of LDL in type IIa hypercholesterolemia

A possible mechanism of action of probucol on low density lipoprotein uptake was examined in 7 type IIa hypercholesterolemic subjects. Probucol administration effectively lowered plasma cholesterol. Both apo B-associated cholesterol and HDL cholesterol were decreased but a great inter-patient variability was noted. Plasma triglycerides were unchanged and phospholipids decreased. The composition of the isolated LDL was unaffected. The LDL displaceable activity of reference [125I]LDL measured by competition assays in control fibroblasts, was increased in 3 subjects, decreased in 1 subject and unchanged in the other 3. No correlation was found between the change in apo B-associated cholesterol and the change in the in vitro catabolism of LDL of treated patients. The results did not allow a simple mechanism of action to be ascribed to the drug, but questioned the origin of the hypercholesterolemia.

Lipoprotein fractions and receptors: a role for probucol?

Cholesterol transport in the plasma involves several lipoprotein families. The process is remarkably ordered and is driven vectorily by apolipoproteins, which activate appropriate enzymes or serve as recognition sites for lipoprotein receptors. Of the lipoproteins in plasma, low density lipoprotein (LDL) contains most of the cholesterol and has the greatest atherogenic potential. Its plasma concentration is determined by LDL receptor activity, which serves to regulate intracellular cholesterol concentrations. LDL receptor activity in the body is not fixed, but can be stimulated by drugs that affect hepatic cholesterol content, such as inhibitors of HMG-CoA reductase or bile acid sequestrants. By stimulating LDL receptor activity, these drugs increase the fractional catabolic rate of apoLDL. HMG-CoA reductase inhibitors also appear to reduce the apoLDL synthetic rate. As a consequence, LDL cholesterol levels are reduced while high density lipoprotein (HDL) cholesterol levels remain stable or increase. Probucol is a drug that lowers both LDL and HDL cholesterol levels. It appears to lower LDL cholesterol levels by affecting LDL structure rather than by stimulating LDL receptor activity. It has no consistent effect on LDL synthetic rates. Probucol lowers plasma HDL cholesterol levels by decreasing the synthetic rates of the major HDL apolipoproteins. The biologic significance of these probucol-induced changes in HDL metabolism is unknown.

Studies on the mechanism of action of probucol

Earlier clinical studies suggested that probucol lowers plasma levels of low density lipoprotein (LDL) by increasing the rate of its removal. The drug has been shown to be effective in patients lacking the LDL receptor and, more recently, in LDL receptor-deficient rabbits. These latter studies showed that probucol increased the fractional catabolic rate of LDL by 40% to 50%. This finding in a receptor-deficient model implies that probucol can enhance the removal of LDL cholesterol by alternative pathways. Because the same enhanced removal was seen when LDL from probucol-treated rabbits was injected into untreated rabbits, it appears that the drug somehow alters the metabolic properties of the LDL itself rather than the metabolic behavior of the tissues of treated animals. Whether or not this mechanism explains the action of probucol in man remains to be determined. It has been postulated that the oxidative modification of LDL might contribute to atherogenesis by facilitating lipid accumulation in macrophages (foam cells) and by inhibiting macrophage motility. LDL resists oxidative modification, however, when probucol is added to in vitro incubations or when the LDL itself is isolated from probucol-treated patients. These findings lay the groundwork for evaluating the possible in vivo significance of LDL oxidation.

Effect of probucol treatment on lipoprotein cholesterol and drug levels in blood and lipoproteins in familial hypercholesterolemia

Twelve patients with mild and 3 with severe hypercholesterolemia were stabilized with an isocaloric diet containing less than 300 mg cholesterol daily with a P/S ratio of 1.8, and placebo period of 4 weeks. They were administered 1000 mg probucol daily for 12 weeks, followed by placebo for 6 weeks. In patients with mild disease, a significant cholesterol reduction was achieved in serum, LDL, and HDL (maximum decrease, 17%, 13%, and 31%, respectively). While HDL3 cholesterol was reduced significantly throughout the period (P less than 0.001), HDL2 cholesterol showed a significant decrease only at the 4th week of treatment (P less than 0.001), and returned to basal levels at the 8th and 12th treatment weeks. Serum apo B levels decreased only slightly, but the HDL-apo A-I fall was significant with a reduction in the HDL-CH/HDL-apo A-I ratio throughout the treatment period. In 3 patients with severe disease, cholesterol decrease in serum and in VLDL, LDL and HDL fractions varied, but on the whole was lower than in patients with mild disease. A decrease in VLDL-CH and HDL-CH was present in all 3, but LDL-CH levels were only slightly lowered in 2 patients, and unchanged in the third. Serum probucol levels fell 66% from the 4th to the 12th treatment week, and in parallel, the percentage of lipoprotein-bound drug increased about 2-fold. It is suggested that these changes in pharmacokinetics as well as the cholesterol-lowering effect of the drug may be due to a change in lipoprotein composition or structure.

Double-blind, placebo-controlled, cross-over trial of probucol in heterozygous familial hypercholesterolaemia

Ten patients with heterozygous familial hypercholesterolaemia (FH) were given probucol 500 mg b.d. or placebo for 3 months in a randomised order in a double-blind cross-over trial. There was a 14% decrease in serum cholesterol concentration due to a reduction in both low density lipoprotein (LDL) cholesterol and high density lipoprotein (HDL) cholesterol. The subfraction of HDL most affected was HDL2. Reductions in serum LDL cholesterol concentration exceeding 20% were obtained in 3 (30%) of the patients. The magnitude of the change in LDL cholesterol concentration was related to the level of serum HDL2 cholesterol without therapy and to the magnitude of its decrease on probucol. Intravenous intralipid tolerance was unaffected by probucol administration. Serum apolipoprotein B concentration decreased less with probucol than did that of serum LDL cholesterol.

Hypocholesterolemic effects of mevinolin in patients with heterozygous familial hypercholesterolemia

We have evaluated the hypolipidemic effects of mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme in cholesterol biosynthesis in 13 patients with heterozygous familial hypercholesterolemia (FH). Patients were maintained on a low-cholesterol diet and received sequentially increasing doses of 5, 10, 20, and 40 mg of mevinolin twice daily for a period of 1 mo on each dose. Plasma concentrations of low density lipoprotein cholesterol decreased by 19.8% on the 5 mg twice daily dose (P less than 0.05 vs. base line), 28.4% on 10 mg of mevinolin twice daily (P less than 0.05 vs. 5 mg twice daily), 35% on 20 mg of mevinolin twice daily (P less than 0.05 vs. 10 mg twice daily), and 37.7% on 40 mg of mevinolin twice daily (not statistically different from 20 mg twice daily). Concentrations of high density lipoprotein cholesterol remained stable on all doses of mevinolin whereas plasma triglyceride levels fell significantly on the 20 mg (-30.7%) and 40 mg (-34.3%) twice daily doses of mevinolin. Mevinolin was well tolerated and all patients completed the study period. Side effects during the period of study were limited to transient insomnia and headaches in two patients, transient increases in alkaline phosphatase in three patients, and a modest but sustained increase in alkaline phosphatase in a fourth patient. These results indicate that mevinolin is an effective hypolipidemic agent in patients with heterozygous FH but that the optimal doses in these patients are greater than those previously reported in normal volunteers. If long-term safety can be satisfactorily established, mevinolin offers considerable promise in the therapy of heterozygous FH.

Effects of probucol on the in vivo plasma clearance of human low density lipoproteins in rabbits and on the expression of lipoprotein receptors in vitro

The purpose of this study was to investigate the effects of probucol on the plasma levels of low density lipoproteins in rabbits and whether the resulting decrease of low density lipoproteins was related to the effects of probucol on the expression of lipoprotein receptors. Probucol administration effectively lowered plasma cholesterol in normal rabbits. Both low density and high density lipoprotein cholesterol decreased, as well as apo B in the former fraction. Probucol had no effect on the fractional catabolic rate of low density lipoprotein while the flux of this lipoprotein decreased to about 50%. Moreover both the binding of lipoproteins to liver membranes and the in vitro uptake of low density lipoprotein by human skin fibroblasts were not affected by the drug. These findings are consistent with an effect of probucol on low density lipoprotein synthesis.