Cholestyramine and ileal by-pass in the treatment of familial hypercholesterolaemia

Inhibition of cholesterol absorption: targeting the intestine

Atherosclerosis, the gradual formation of a lipid-rich plaque in the arterial wall is the primary cause of Coronary Artery Disease (CAD), the leading cause of mortality worldwide. Hypercholesterolemia, elevated circulating cholesterol, was identified as a key risk factor for CAD in epidemiological studies. Since the approval of Mevacor in 1987, the primary therapeutic intervention for hypercholesterolemia has been statins, drugs that inhibit the biosynthesis of cholesterol. With improved understanding of the risks associated with elevated cholesterol levels, health agencies are recommending reductions in cholesterol that are not achievable in every patient with statins alone, underlying the need for improved combination therapies. The whole body cholesterol pool is derived from two sources, biosynthesis and diet. Although statins are effective at reducing the biosynthesis of cholesterol, they do not inhibit the absorption of cholesterol, making this an attractive target for adjunct therapies. This report summarizes the efforts to target the gastrointestinal absorption of cholesterol, with emphasis on specifically targeting the gastrointestinal tract to avoid the off-target effects sometimes associated with systemic exposure.

Three Generations of Bile Acid Sequestrants

Cholestyramine, the first bile acid sequestrant to be marketed, has been in use for over 20 years. Despite its low potency, requiring 16-24 g of polymer to achieve 20% LDL cholesterol reduction in hypercholesterolemic individuals, only one other sequestrant, colestipol, has come to market in the ensuing period. GelTex Pharmaceuticals has been involved for over six years in the discovery and development of new, more potent polymeric sequestrants. Two binding mechanisms are presented — one that operates via an aggregate binding structure and one that is effective via a defined site binding structure. These two binding mechanisms are compared and contrasted through bile acid binding isotherms. The best of these new sequestrants bind bile acids through a combination of hydrophobicity and ion exchange. Optimization and balancing of each of these interactions led us to more potent materials. The first of these, colesevelam hydrochloride is expected to be three to four times more potent than cholestyramine. A third generation product is still in research at GelTex. With another twofold increase in potency possible, single tablet therapy may become a reality.

The pharmacological exploitation of cholesterol 7alpha-hydroxylase, the key enzyme in bile acid synthesis: from binding resins to chromatin remodelling to reduce plasma cholesterol

Mammals dispose of cholesterol mainly through 7alpha-hydroxylated bile acids, and the enzyme catalyzing the 7alpha-hydroxylation, cholesterol 7alpha-hydroxylase (CYP7A1), has a deep impact on cholesterol homeostasis. In this review, we present the study of regulation of CYP7A1 as a good exemplification of the extraordinary contribution of molecular biology to the advancement of our understanding of metabolic pathways that has taken place in the last 2 decades. Since the cloning of the gene from different species, experimental evidence has accumulated, indicating that the enzyme is mainly regulated at the transcriptional level and that bile acids are the most important physiological inhibitors of CYP7A1 transcription. Multiple mechanisms are involved in the control of CYP7A1 transcription and a variety of transcription factors and nuclear receptors participate in sophisticated regulatory networks. A higher order of transcriptional regulation, stemming from the so-called histone code, also applies to CYP7A1, and recent findings clearly indicate that chromatin remodelling events have profound effects on its expression. CYP7A1 also acts as a sensor of signals coming from the gut, thus representing another line of defence against the toxic effects of bile acids and a downstream target of agents acting at the intestinal level. From the pharmacological point of view, bile acid binding resins were the first primitive approach targeting the negative feed-back regulation of CYP7A1 to reduce plasma cholesterol. In recent years, new drugs have been designed based on recent discoveries of the regulatory network, thus confirming the position of CYP7A1 as a focus for innovative pharmacological intervention.

1-[4-[4[(4R,5R)-3,3-Dibutyl-7-(dimethylamino)-2,3,4,5-tetrahydro-4-hydroxy-1,1-dioxido-1-benzothiepin-5-yl]phenoxy]butyl]-4-aza-1-azoniabicyclo[2.2.2]octane methanesulfonate (SC-435), an ileal apical sodium-codependent bile acid transporter inhibitor alters hepatic cholesterol metabolism and lowers plasma low-density lipoprotein-cholesterol concentrations in guinea pigs

Male Hartley guinea pigs (10/group) were assigned either to a control diet (no drug treatment) or to diets containing 0.4, 2.2, or 7.3 mg/day of an ileal apical sodium-codependent bile acid transporter (ASBT) inhibitor, 1-[4-[4[(4R,5R)-3,3-dibutyl-7-(dimethylamino)-2,3,4,5-tetrahydro-4-hydroxy-1,1-dioxido-1-benzothiepin-5-yl]phenoxy]butyl]-4-aza-1-azoniabicyclo[2.2.2] octane methanesulfonate (SC-435). Based on food consumption, guinea pigs received 0, 0.8, 3.7, or 13.4 mg/kg/day of the ASBT inhibitor. The amount of cholesterol in the four diets was maintained at 0.17%, equivalent to 1200 mg/day in the human situation. Guinea pigs treated with 13.4 mg/kg/day SC-435 had 41% lower total cholesterol and 44% lower low-density lipoprotein (LDL)-cholesterol concentrations compared with control (P < 0.01), whereas no significant differences were observed with either of the lower doses of SC-435. Hepatic cholesterol esters were significantly reduced by 43, 56, and 70% in guinea pigs fed 0.8, 3.7, and 13.4 mg/kg/day of the ASBT inhibitor, respectively (P < 0.01). In addition, the highest dose of the inhibitor resulted in a 42% increase in the number of very low-density lipoprotein (VLDL) triacylglycerol molecules and a larger VLDL diameter compared with controls (P < 0.05). Acyl-CoA cholesterol/acyltransferase activity was 30% lower with the highest dose treatment, whereas cholesterol 7alpha-hydroxylase, the regulatory enzyme of bile acid synthesis, was 30% higher with the highest ASBT inhibitor dose (P < 0.05). Furthermore, bile acid excretion increased 2-fold with the highest dose of SC-435 compared with the control group (P < 0.05). These results suggest that the reduction in total and LDL-cholesterol concentrations by the ASBT inhibitor is a result of alterations in hepatic cholesterol metabolism due to modifications in the enterohepatic circulation of bile acids.

Effects of soybean protein and very low dietary cholesterol on serum lipids, biliary lipids, and fecal sterols in humans

Soy-base texturized vegetable protein (TVP; Archer Daniels Midland, Decatur, IL) has been used to decrease serum cholesterol and as a substitute for animal protein to achieve very low levels of dietary cholesterol. The effect of very low dietary cholesterol and of TVP on biliary lipids and fecal sterols is unclear. The study objective was to determine the effects of very low intake of dietary cholesterol, as well as TVP itself, on serum lipids, biliary lipids, and fecal sterols. We studied eight normal subjects living on a metabolic ward during three randomly ordered 6- to 7-week periods: (1) standard cholesterol diet (190 to 550 mg/d), (2) TVP-low-cholesterol diet (17 to 30 mg/d), and (3) TVP-standard cholesterol diet. By analysis of covariance (ANCOVA), reducing dietary cholesterol to these very low levels significantly decreased serum low-density lipoprotein (LDL) cholesterol (P=.048) but did not affect high-density lipoprotein (HDL) cholesterol or triglyceride. TVP resulted in a borderline significant reduction in LDL cholesterol (P=.058) with a highly significant reduction in HDL cholesterol (P=.004) and an increase in serum triglyceride (P=.010). During TVP ingestion, there was a highly significant increase in the output of fecal neutral sterols (P=.005) and a tendency for a higher output of fecal acidic sterols (P=.100). Fecal sterol balance was significantly more negative (indicating increased cholesterol synthesis) during TVP ingestion (P=.016). Neither TVP nor the very-low-cholesterol diet appreciably affected the gallbladder bile molar percent cholesterol or saturation index. The data are consistent with the hypothesis that to the extent TVP decreases serum LDL cholesterol (an effect of borderline significance in this study), the effect occurs via a reduction in the absorption of cholesterol and perhaps bile acid. However, the potential benefit of decreasing LDL cholesterol in this way seems to be at least partially offset by a concomitant reduction in HDL cholesterol and an increase in serum triglycerides.

Cholesterol lowering and bile acid excretion in the hamster with cholestyramine treatment

Cholestyramine was administered to hamsters at 6 doses in the diet for 1 week. Plasma cholesterol, LDL + VLDL cholesterol and HDL cholesterol were measured after this period. Bile acid excretion was measured in faeces collected over the final 24 h of the experiment. A dose-response curve for each parameter measured was constructed using data from individual hamsters. For the bile acid and the cholesterol measurements a maximum response was observed at the highest doses. A correlation between the bile acids excreted over 24 h and the LDL + VLDL cholesterol showed that the maximum effect of cholestyramine on lowering plasma and lipoprotein cholesterol occurred at a submaximal excretion level of bile acids. Comparison of the efficiency of cholestyramine in reducing plasma cholesterol in the hamster with limited data in the dog and in man suggest that a greater lowering of plasma cholesterol is achieved in the dog and in man for an equivalent increase in bile acid excretion caused by the sequestrant. As is already known, cholestyramine treatment caused an increase in hepatic cholesterol 7 alpha-hydroxylase and HMG-CoA reductase activity. Interestingly in this study the novel observation was made that the bile acid sequestrant reduced the activity of hepatic acyl-CoA: cholesterol acyltransferase.

The hyperlipoproteinemias

The association of disturbances of plasma lipid transport and atherogenesis has been recognized, and scientific data continue to accumulate to explain this association from a mechanistic viewpoint. A number of recent clinical trials have shown that cholesterol-lowering therapy can prevent the complications of atherosclerosis. Consequently, the attention of physicians to therapeutic intervention has increased and public awareness to plasma cholesterol levels has been heightened. This article summarizes current knowledge of how plasma lipid transport is regulated. The classical primary hyperlipoproteinemias are considered and hyperlipoproteinemias occurring secondary to other diseases are discussed. Standard methods to diagnose the defined genetic hyperlipidemias are outlined, and new approaches to assess risk of atherosclerosis are examined. Finally, the role of dietary measures and drugs in lowering blood lipids and reducing risk of coronary heart disease is delineated.

Plasma and biliary cholestanol related to steroid metabolism in familial hypercholesterolemia patients with and without ileal exclusion

Plasma cholestanol is increased in cerebrotendinous xanthomatosis and in sitosterolemia with xanthomatosis. We measured plasma and biliary cholestanol in heterozygous familial hypercholesterolemia patients with (n = 10) and without (n = 12) ileal exclusion. In the unoperated patients plasma cholestanol concentration (12.9 mumol/l) and content (1.2 mmol/mol cholesterol) were slightly higher than in the nonhypercholesterolemic control subjects studied by us. Ileal exclusion had lowered plasma cholestanol concentration but only in proportion to the lowering of plasma cholesterol concentration, and plasma cholestanol content (mmol/mol cholesterol) was similar in the operated and unoperated subjects. Plasma and biliary cholestanol contents were positively associated. In the unoperated patients the fractional cholesterol absorption and plasma plant sterols, also reflecting sterol absorption, were positively correlated with plasma cholestanol content. Our study suggests, that plasma cholestanol is slightly elevated in familial hypercholesterolemia and that, in addition to plasma lipoprotein level, sterol absorption is important in the regulation of plasma cholestanol level. Ileal exclusion decreases plasma cholestanol in proportion to the decrement in the plasma cholesterol concentration.

Effect of ileal exclusion on lipoprotein sitosterol in familial hypercholesterolaemia

A plant sterol, sitosterol, was quantitated in very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL) and related to faecal steroids and cholesterol absorption in heterozygous familial hypercholesterolaemia patients with (n = 7) and without ileal bypass (n = 6). The latter had resulted in severe bile acid malabsorption but fractional cholesterol absorption was within low control limits. Serum total and LDL cholesterol and apoprotein B levels were reduced, whereas HDL cholesterol, apoprotein A-I, VLDL and HDL sitosterol concentrations were increased by the ileal exclusion, and the increase in LDL and serum total sitosterol levels was insignificant. In terms of mmol/mol of cholesterol or apoprotein B, however, the LDL and total sitosterol contents were higher in the subjects who had undergone operation. For an unknown reason the sitosterol content increased gradually within the lipoprotein particles from the lighter to the heavier lipoproteins, and the enrichment was similar in the two groups. Dietary sitosterol intake, indicated by faecal sitosterol excretion, was similar in the two groups. The contents of serum total and LDL sitosterol were positively correlated with the dietary sitosterol intake in both groups, and with the fractional cholesterol absorption only in the group not subject to operation. These associations were less consistent for sitosterol contents in other lipoproteins. We conclude that normally the serum sitosterol content reflects cholesterol absorption efficiency even in patients with familial hypercholesterolaemia, provided the dietary sitosterol intake is quite constant. In addition, for unknown reasons ileal exclusion leads to an increased lipoprotein sitosterol content.

Peripheral arterial disease in heterozygous familial hypercholesterolemia: no difference between patients with and without partial ileal bypass

We evaluated lower limb arterial circulation in 37 patients with heterozygous familial hypercholesterolemia using strain-gauge plethysmography. Nineteen patients were initially allocated to surgical therapy (ileal bypass operation), and 18 control patients were treated conservatively. After a follow-up period of 3-12 years (mean 10 yrs) the surgically treated patients had significantly lower serum total (9.4 vs. 11.9 mmol/l, P less than 0.001) and LDL (6.2 vs. 8.1 mmol/l, P less than 0.01) cholesterol and apoprotein B levels and higher HDL cholesterol (1.2 vs. 1.0 mmol/l, P less than 0.05) and apoprotein A-I levels compared with controls. There was no significant difference in the prevalence of intermittent claudication (16 vs. 6%) or coronary heart disease (68 vs. 61%) between the operated and control groups. The ankle-arm systolic blood pressure ratio was pathologically low in 53% of the operated and 72% of the control patients, and the toe-arm pressure ratio in 50% and 44%, respectively. These differences or the differences in the mean ankle-arm and toe-arm pressure ratios between the groups were not significant. The toe-ankle pressure gradient was normal in all but 2 patients indicating that atherosclerotic changes predominantly affect the more proximal arteries in familial hypercholesterolemia. Patients with peripheral arterial disease could not be differentiated by serum lipoprotein levels, blood pressure or the presence of coronary heart disease. However, smoking was more prevalent (38% vs. 0%, P less than 0.05) in patients with impaired peripheral circulation. We conclude that asymptomatic peripheral arterial disease is quite common in familial hypercholesterolemia, and that smoking increases its risk.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol absorption regulates cholesterol metabolism and plasma lipoprotein levels in patients with gut exclusions

The occurrence of cholesterol malabsorption and its role in the regulation of cholesterol metabolism were studied in 30 patients with an earlier gut resection and 9 patients with a jejunoileal bypass for treatment of obesity. Fractional cholesterol absorption varied from 0.1% to 70%, and was lowest in jejunoileal bypass (8.3%) associated with severe fat and moderate bile acid malabsorptions and in 15 patients with a long small-intestinal resection (20.4%) associated with severe bile acid and moderate fat malabsorption. Seven resected patients with normal fecal fat and bile acids and 8 resected patients with malabsorption of only bile acid had normal cholesterol absorption. Low fractional cholesterol absorption was associated with a short length of the remaining proximal small intestine, high dietary intake of plant sterol, and high fecal fat and neutral sterol excretions, but not with bile acid malabsorption. In the whole study population, plasma levels of total, low-density lipoprotein, and high-density lipoprotein cholesterol were positively correlated with fractional cholesterol absorption and the amount of total, dietary, and biliary absorbed cholesterol and were negatively correlated with fecal cholesterol elimination as neutral sterols (less so as bile acids) and cholesterol synthesis. The results emphasize that, in patients with ileal exclusion, plasma levels of low-density lipoprotein and high-density lipoprotein cholesterol are regulated more effectively by cholesterol than by bile acid malabsorption. Moreover, although the fecal loss of bile acids is the main determinant in cholesterol elimination and stimulation of cholesterol synthesis in patients with intestinal exclusions, intestinal cholesterol absorption also contributes noticeably to the regulation of cholesterol synthesis.

Fecal bile acids related to small-bowel length before and after ileal exclusion

We studied the relationship among the small-bowel length, the fecal excretion of bile acids and neutral steroids, and cholesterol synthesis in 16 patients with familial hypercholesterolemia before and 1 month after the ileal bypass operation. The mean small-bowel length measured during the operation was 6.3 +/- 0.3 (SEM) m before and 4.3 +/- 0.2 m after the bypass of the distal third of the small bowel. The preoperative fecal bile acid excretion was negatively correlated (r = 0.63, p less than 0.01) with the small-bowel length, whereas this correlation was insignificant for fecal neutral steroid excretion and cholesterol synthesis. Postoperative amounts of fecal bile acids and neutral steroids and cholesterol synthesis were negatively correlated with the length of the small bowel remaining in function (r = -0.52, -0.51, and -0.62, respectively). Our study suggests that fecal bile acid excretion and, hence, bile acid synthesis are related to the small-bowel length even under physiologic conditions.

High density lipoprotein, apoproteins A-I and A-II and postheparin plasma lipolytic enzymes after ileal bypass

Apoproteins A-I and A-II, and the activities of lipoprotein lipase (LPL) and hepatic lipase (HL), were studied in 16 patients 3-12 years after ileal bypass operation and in 13 controls, all heterozygous for familial hypercholesterolemia, to investigate why the operated subjects had a higher HDL cholesterol level than the unoperated controls. HDL- and HDL2-cholesterol and apoprotein A-I were higher, HDL3-cholesterol was similar and apoprotein A-II tended to be lower in the operated than the control subjects. The activities of LPL and HL were similar in the 2 groups. HL was negatively correlated with HDL2-cholesterol, whereas LPL was not associated with any of the HDL components. The controls had gained in weight during the follow-up, but the HDL components were not correlated with relative body weight. It is concluded that in familial hypercholesterolemia ileal bypass results in higher HDL- and HDL2-cholesterol and apoprotein A-I level than conservative treatment and that postheparin plasma lipolytic enzymes do not explain the higher level of these HDL components in the operated subjects.

Effect of ileal exclusion on kinetics of very low density lipoprotein triglycerides in familial hypercholesterolemia

Plasma lipoproteins, VLDL triglyceride kinetics, and bile acid and cholesterol synthesis were measured in 21 patients heterozygous for familial hypercholesterolemia with (n = 11) or without (n = 10) ileal bypass. LDL cholesterol and apoprotein B concentrations were lower, and cholesterol and bile acid synthesis, the VLDL triglyceride/cholesterol ratio, and the HDL cholesterol concentration were higher in the operated than the control patients. The VLDL triglyceride production rate was increased in the operated normotriglyceridemic patients by about 65%, whereas the fractional catabolism of VLDL triglycerides and the calculated VLDL cholesterol transport were similar in the operated and control groups. VLDL triglyceride production was not correlated with cholesterol or bile acid synthesis. The VLDL triglyceride concentration was positively correlated with the production and negatively with the fractional catabolism of VLDL triglycerides. In unoperated normotriglyceridemic patients the VLDL triglyceride production was positively correlated with LDL cholesterol (r = 0.69, p less than 0.05), LDL triglyceride (r = 0.84, p less than 0.01) and LDL apoprotein B (r = 0.80, p less than 0.01) concentrations, and with the LDL triglyceride/apoprotein B (r = 0.72, p less than 0.05) and LDL triglyceride/cholesterol (r = 0.68, p less than 0.05) ratios. None of these correlations was significant in the operated patients. We conclude that in heterozygous familial hypercholesterolemia VLDL triglyceride level depends on both VLDL triglyceride synthesis and catabolism, LDL level is proportionate to VLDL triglyceride production in the unoperated patients but not in the patients with ileal bypass, ileal exclusion results in an increase in the production rate of VLDL triglycerides in normotriglyceridemic patients but otherwise VLDL triglyceride production is poorly associated with cholesterol and bile acid synthesis, ileal exclusion may induce hepatic secretion of triglyceride-rich VLDL.

Adaptation of cholesterol and bile acid metabolism and vitamin B12 absorption in the long-term follow-up after partial ileal bypass

Cholesterol and bile acid metabolism and vitamin B12 were studied in 19 patients with heterozygous familial hypercholesterolemia in an 8-yr follow-up (3-12 yr after ileal bypass operation), and in 11 unoperated controls. Absorption of cholesterol and vitamin B12 were decreased by the operation, but improved slowly, and at 8 yr cholesterol absorption was normal. Cholesterol excretion as fecal neutral steroids was not increased by the operation, and at 8 yr the flux of endogenous cholesterol to the gut was similar in the operated and control patients. Cholesterol absorption was positively correlated with mouth to anus transit time in the unoperated patients. Fecal bile acid excretion was increased immediately after the operation and continued to increase even after the second postoperative year. In the operated patients fecal excretion of fat, water, and dry matter were positively correlated with fecal bile acid excretion. Our study suggests that adaptive changes occur slowly after ileal bypass, resulting in gradual normalization of cholesterol absorption, despite continuing bile acid malabsorption, and that the intestinal transit time is related to steroid absorption even under physiologic conditions.

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.

Long-term effects of ileal bypass on lipoproteins in patients with familial hypercholesterolemia

Partial ileal bypass is effective in reducing circulating cholesterol levels. In our study the 10 year effects of the procedure on serum lipids and lipoproteins were studied in 27 patients with heterozygous familial hypercholesterolemia. The ileal bypass patients were compared with conservatively treated case controls matched for age, sex, serum cholesterol level, relative body weight, blood pressure, and smoking habits and also for the existence of diabetes and coronary heart disease. Serum triglycerides were initially slightly higher in the patients undergoing ileal bypass. During the 10 year follow-up eight surgically treated and seven control patients suffered fatal or nonfatal myocardial infarctions. Of these all but one male subject who underwent surgery had had manifest coronary heart disease at entry. Male sex, smoking, triglyceride levels, and angina were significant predictors of new coronary events. The fall in serum cholesterol in patients who underwent ileal bypass and had fatal myocardial infarctions was smaller than in the corresponding subjects without events. The serum lipid levels of the survivors at the end of the 10 year follow-up showed that ileal bypass, as compared with the conservative treatment, had led to a larger decrease in total serum cholesterol (-33% vs -11% in the control patients; p less than .001); lower total serum cholesterol (360 vs 468 mg/dl; p less than .001), low-density lipoprotein (LDL) cholesterol (236 vs 324 mg/dl, p less than .001), and LDL apoprotein B levels (186 vs 231 mg/dl; p less than .001); and higher serum high-density lipoprotein (HDL) (46 vs 38 mg/dl; p less than .05) and HDL2 cholesterol levels (25 vs 16 mg/dl; p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Fat absorption and metabolism

This review spans almost 20 years during which the author's initial interest in the hypolipidaemia of patients with steatorrhoea eventually led him to treat hyperlipidaemia by deliberately inducing malabsorption of cholesterol and bile acids. Before discussing malabsorption, however, it is necessary to first consider the normal physiology of fat absorption and metabolism. This process can be divided into various phases as summarised below: The intraluminal phase, involves lipolysis and micellar solubilisation of dietary fat; The mucosal phase, involves uptake of micellar lipids and their subsequent re-esterification; The lymphatic phase, involves chylomicron formation and secretion into lymph; and The catabolic phase, which involves the peripheral hydrolysis of chylomicrons, the uptake of remnant particles by the liver and the subsequent recycling and exchange of lipids and apoproteins among the various plasma lipoproteins.

Turnover of low density lipoproteins during inhibition of cholesterol absorption by neomycin

The mechanisms for the hypocholesterolemic actin of neomycin were examined in six patients with various levels of plasma total cholesterol and triglycerides. All patients were studied on a metabolic ward. The first period of 6 weeks was for control. Thereafter, neomycin (1.5 g/day) was started, and the patients were readmitted for another 6-week period after 2 to 3 months of treatment with the drug. Cholesterol balance studies showed that neomycin increased fecal excretion of neutral steroids by an average of 45%; the drug also inhibited absorption of exogenous cholesterol by an average of 44%. During treatment with neomycin, the plasma total cholesterol fell by an average of 20%, low density lipoproteins (LDL) fell by 25%, and high density lipoproteins, by 16%. Neomycin did not change plasma triglyceride levels. Turnover of the apoprotein of LDL (apoLDL) was measured following injection of 125I-apoLDL. Neomycin decreased synthesis of apoLDL by 28%. The decrease in plasma apoLDL level was correlated positively with the decrease in apoLDL synthetic rate. The effect of the drug on clearance of LDL was less constant; four of six patients had an increase in fractional clearance rates of apoLDL, but the change for the whole group was not statistically significant. These data suggest that a decrease in production of LDL is a major factor in the lowering of LDL following inhibition of cholesterol absorption; however, an increase in clearance rates may occur in some patients.

Effects of hypolipidemic therapy on cholesterol homeostasis in freshly isolated mononuclear cells from patients with heterozygous familial hypercholesterolemia

Freshly isolated mononuclear leukocytes have been reported to show changes in cholesterol synthesis and high-affinity degradation of low-density lipoproteins (LDL) that parallel those that occur in the liver. To examine whether hypolipidemic therapy in patients with heterozygous familial hypercholesterolemia influences cholesterol homeostasis in their mononuclear cells we assessed the effects of colestipol and nicotinic acid (alone and in combination) on the rates of high-affinity 125I-labeled LDL degradation and on the rates of cholesterol and phosphatidylcholine biosynthesis by freshly isolated cells. Rates of 125I-labeled LDL degradation were lower in mononuclear cells from patients with heterozygous familial hypercholesterolemia on no medication (3.1 ng per 4 X 10(6) cells per 5 hr) than in cells from normal control subjects (6.1 ng per 4 X 10(6) cells per 5 hr) and, in the former patients, the values were not significantly affected by therapy with nicotinic acid. In contrast, freshly isolated mononuclear cells from patients receiving colestipol degraded 125I-labeled LDL at near-normal rates (5.0 ng per 4 X 10(6) cells per 5 hr). The rates of cholesterol synthesis were also higher in mononuclear cells isolated from patients treated with colestipol than in cells from untreated patients or from those receiving nicotinic acid; in contrast the rate of synthesis of phosphatidylcholine did not show any consistent changes. Similar results were obtained in a smaller number of patients studied longitudinally, in which colestipol therapy significantly increased rates of cholesterol synthesis and high-affinity degradation of 125I-labeled LDL by freshly isolated mononuclear cells. We conclude that previously observed changes in cholesterol homeostasis in the liver of patients treated with bile acid sequestrants are paralleled by similar changes in freshly isolated mononuclear cells and that these cells offer an accessible model for further studies on how diet and pharmacologic agents influence cellular cholesterol homeostasis in humans.

Partial ileal bypass inhibits atherosclerosis in WHHL rabbits

The effectiveness of partial ileal bypass (PIB) as a counter-measure against atherosclerosis was evaluated in WHHL rabbits. The effects of PIB and sham operation, each performed in five animals, on serum lipids, lipoproteins and plaque formation were investigated. PIB resulted in an immediate and sustained decrease of 52% (range 29-67%) in serum cholesterol, while sham operation had no effect. The main reduction was in LDL cholesterol; VLDL-cholesterol was lowered to a lesser extent. PIB also appeared to change the electrophoretic behaviour of total serum, very low density and low density lipoproteins. Plaque formation, measured 30 weeks after operation in various aortic segments and arteries, was significantly reduced after PIB. It is concluded that an induced lowering of serum cholesterol can prevent atherosclerosis in WHHL rabbits. Also, these animals must be considered as a model for the receptor-defective cellular phenotype of homozygous familial hypercholesterolemia, not for the receptor-negative type, which is the only truly genetically homozygous form.

Partial Ileal Bypass in Treatment of Familial Hypercholesterolemia

The effects of partial ileal bypass have been studied in 7 patients with heterozygous familial hypercholesterolaemia. By the seventh to ninth postoperative day there was a significant fall in plasma cholesterol of 42.8% compared with levels at initial presentation. Thereafter, plasma levels of cholesterol rose by varying degrees, but had stabilized by 3 months with average values 29.5% lower than pretreatment. Debilitating diarrhoea or severe weight loss were not troublesome features of the operation in most patients.

The effect of partial ileal bypass on receptor-mediated uptake and catabolism of low density lipoprotein in the rhesus monkey

The fractional rates of catabolism (FCR) of 125I-labelled low-density lipoprotein (LDL) and 131I-labelled LDL coupled with 1,2-cyclohexanedione (LDL-CHD) were determined before and 6 weeks after partial ileal bypass in three Rhesus monkeys. Receptor-mediated catabolism, determined as the difference between the FCRs of 125I-LDL and 131I-LDL-CHD, increased significantly after partial ileal bypass. Analysis of tissue distribution of radioactivity in one monkey, killed at the conclusion of a third turnover study 6 months after partial ileal bypass, suggested that the liver was the main tissue site of accumulation of LDL, and that uptake had occurred mainly via a receptor-mediated mechanism. It is concluded that partial ileal bypass stimulates receptor-mediated LDL catabolism in the Rhesus monkey as in man, and that this increase is probably mediated via hepatic LDL receptors.

Effects and side-effects of partial ileal by-pass surgery for familial hypercholesterolaemia

Ten patients with familial hypercholesterolaemia were subjected to partial ileal by-pass surgery. Plasma cholesterol fell by 41 and 38% and low-density lipoprotein cholesterol by 51 and 46% after six and 18 months respectively. High-density and very low-density lipoprotein cholesterol and plasma triglycerides were unaffected. Alanine aminotransferase increased transiently in half of the patients. Diarrhoea and slight steatorrhoea troubled most of the patients for the duration of 18 months' period of observation. Other long-term side effects were slight but significant increase in the renal excretion of oxalic acid and reduction in the intestinal absorption of calcium. The study shows that this operation has metabolic side-effects that warrant continued medical care of these patients.

Receptor-mediated low-density lipoprotein catabolism

Low-density lipoprotein (LDL) receptors are demonstrable in cultured fibroblasts from normal subjects but are decreased or absent in cells from patients with heterozygous or homozygous familial hypercholesterolaemia. In vivo receptor-mediated LDL catabolism, determined as the difference between the turnover rates of 125I-LDL and 131I-LDL coupled with cyclohexanedione, is responsible for approximately one-third of the total catabolism of LDL in normal subjects, but less than one-fifth in heterozygotes and is totally absent in homozygotes. Receptor-mediated catabolism can be stimulated in normal subjects and in heterozygotes by measures that promote bile acid synthesis, namely, administration of anion-exchange resins or creating a partial ileal bypass. Studies in dogs have shown that such measures stimulate the high-affinity binding of LDL by liver cell membranes. Taken together, these observations suggest the existence of LDL receptors in human liver, the function of which is to maintain cholesterol homeostasis within the hepatocyte during periods of increased demand. Partial or complete absence of such hepatic receptors may play a major role in the pathogenesis of familial hypercholesterolaemia.

Treatment of hypercholesterolemia

After discussing the indications for treatment of familial hypercholesterolemia and the importance of a differential diagnosis, the authors describe drug therapy for the disorder with special attention to combined drug regimens. The surgical treatment of hypercholesterolemia and the treatment of homozygous and other forms of hypercholesterolemia are also detailed.

Superiority of partial ileal bypass over cholestyramine reducing cholesterol in familial hypercholesterolaemia

Low density lipoprotein (LDL) turnover studies were conducted on three occasions in each of six patients with heterozygous familial hypercholesterolaemia (FH)-on diet, after 1 month on cholestyramine 16 g/day, and 2 months after partial ileal bypass. Partial ileal bypass lowered total and LDL cholesterol levels and increased the fractional catabolic rate of LDL to a greater extent than did cholestyramine; this difference presumably reflects the greater increase in bile acid excretion induced by the surgical procedure. Studies in two other heterozygotes showed that partial ileal bypass specifically enhanced receptor-mediated catabolism of LDL. The findings support the concept that therapeutic stimulation of bile acid synthesis increases the rate of receptor-mediated uptake and degradation of LDL by the liver.

Normalization of low-density-lipoprotein levels in heterozygous familial hypercholesterolemia with a combined drug regimen

We studied the effect of th bite acid sequestrant colestipol, alone and in combination with clofibrate or niacin, in patients with heterozygous familial hypercholesterolemia who were given a diet low in cholesterol and saturated fat. With colestipol alone, mean cholesterol levels in serum decreased 16 to 25 per cent. The addition of clofibrate produced a total mean decrement of only 28 per cent. In contrast, serum cholesterol levels fell 45 per cent when colestipol as combined with niacin. Low-density-lipoprotein (LDL) cholesterol decreased 55 per cent with colestipol and niacin, whereas high-density-lipoprotein (HDL) cholesterol increased. Mean LDL cholesterol was lower in patients given this regimen than in matched normal controls eating an unrestricted diet. Tendinous xanthomas, measured by quantitative xeroradiography, were significantly reduced (P < 0.01), indicating that this regimen mobilized cholesterol from tissue pools with slow turnover. Colestipol plus niacin promises to be useful in the treatment of patients at high risk from elevated levels of LDL.