Featured Commentary

Issue No. 4: June, 2011

New in the literature

Anti-thrombotic effects of n-3 fatty acids

Two recent studies focus on anti-thrombotic effects of n-3 fatty acids, from marine or plant-derived sources

The first, in the clinical setting of stable coronary artery disease patients undergoing percutaneous coronary intervention (PCI), show that adding n-3 fatty acids (1 g/day) to standard therapy improves clot properties and decreases thrombin formation. These actions may contribute to a decrease in thrombotic effects in patients.³

The OMEGA-PCI Clot study randomised 54 patients (24% female, mean age 63 years) to treatment with n-3 polyunsaturated fatty acids or placebo, in addition to standard pharmacotherapy including aspirin and clopidogrel. Treatment was initiated before PCI and continued for 1 month. The effect of treatment on fibrin clots was assessed using functional assays, measuring fibrin clot permeability, lysis time, prothrombin fragment 1.2, peak thrombin generation, 8-isoprostaglandin F_2α (a measure of oxidative stress) and C-reactive protein at baseline, and at 3-5 days and 30 days after randomization.

Treatment with n-3 fatty acids resulted in a less compact clot which was more susceptible to fibrinolysis. Prothrombin fragment, peak thrombin generation and 8 isoprostaglandin F_2α were all significantly lower at 30 days after treatment; however, there was no effect on either fibrinogen or C-reactive protein. Thus, the change in clot characteristics cannot be explained by an effect on fibrinogen, a key modulator of fibrin clot structure. Instead, improved clot characteristics may be attributable, at least partly, to reduced thrombin and free radical generation, consistent with previous findings. In particular, the reduction in oxidative stress is noteworthy, given that oxidative stress may promote conversion to fibrin and platelet aggregation and detrimentally affect plasminogen activation. It should also be borne in mind that these beneficial changes in clot characteristics occurred despite treatment with agents that enhance clot permeability and lysis.4,5

The authors acknowledged that the number and nature of patients enrolled in the study, and lack of clinical outcomes data, were limitations to wider extrapolation of their findings. Despite this, the study adds new information on novel anti-thromboticproperties associated with n-3 fatty acid supplementation in patients with stable coronary artery disease. Whether these effects translate to clinical benefit awaits further evaluation.

The second experimental study in mice showed that α-linolenic acid (ALA), an essential n-3 fatty acid derived from plants, has important anti-thrombotic effects.⁶ Previous studies have shown that ALA consumption has been associated with clinical benefit, in terms of reduced risk of myocardial infarction (MI) and sudden cardiac death.^7,8 However, as pointed out in the recent EAS Consensus Panel Paper on triglyceride-rich lipoprotein remnants and HDL cholesterol,² some of these studies require replication to confirm the magnitude of benefits observed. Additionally, the biological mechanisms underlying these effects remain poorly understood.

Investigators fed C57B1/6 mice with diets supplemented with either high (7.3%, treated group) or low ALA (0.03%, control group) for 2 weeks. Treated mice showed delayed arterial thrombus formation in response to photochemical injury compared with controls (p<0.005). This effect was associated with reduced platelet activation induced by thrombin and collagen both in vivo (p<0.005) and ex vivo in human platelets treated with ALA (p<0.01).

ALA supplementation also impaired TF expression and activity. Further investigation showed that this occurred at the transcriptional level via the mitogen-activated protein kinase p38 in vascular smooth muscle cells and p38, ERK1/2, and JNK1/2 in endothelial cells. Dietary ALA had no effect on plasma clotting times and thrombin generation.

These findings, although in an experimental model, are strengthened by the fact that ALA concentrations achieved were in the clinically relevant range. Control experiments with other fatty acids (n-6 linolenic acid and the saturated fatty acid stearic acid) had no effect on TF expression. Thus, the authors concluded that the study provides solid evidence for direct potent dual anti-thrombotic effects of an ALA-supplemented diet, which warrant further evaluation in clinical trials. They highlight the potential of plant-derived ALA as an alternative to marine n-3 fatty acids, given increasing pressure on finite fish reserves.

Efficacy of mipomersen in hyperlipidemia

Mipomersen is a novel anti-sense compound that lowers LDL cholesterol by inhibiting apolipoprotein (apo) B production. This agent has attracted attention for management of familial hypercholesterolemia. However, it may also hold promise for further reducing LDL cholesterol in high-risk patients, especially those in whom high-dose statin therapy is limited by side effects. A study in patients with mild to moderate hyperlipidemia (LDL cholesterol levels in the range of 3.1-6.9 mmol/L (119 266 mg/dL) adds further information.⁹

This randomized, placebo-controlled, double-blind, dose-escalation study enrolled 50 patients (45 men and 5 women, aged 28-65 years). At baseline, none were receiving lipid-modifying treatments. Patients were treated over 13 weeks over the dose range 50, 100, 200, 300 or 400 mg/week with or without dose loading.

Treatment with mipomersen was effective in reducing LDL cholesterol and apoB, especially at higher doses, with reductions of ≈45% at 200 mg/week and ≈60% at 300 mg/week (see Figure 1). Mipomersen also reduced other atherogenic apoB-containing lipoproteins including very low-density lipoproteins (VLDL), triglycerides and lipoprotein(a). 50% of patients in the 200 mg/week group and 100% in the 400 mg/week group achieved LDL cholesterol levels <100 mg/dL (≈2.5 mmol/L)

Figure 1. Effects of mipomersen (weekly dosage) on apo-B containing lipoproteins
Data are given as mean except for triglycerides (TG, median values).

Injection site reaction (erythema) was reported in all subjects, but did not worsen with repeated dosing. Elevation in liver enzymes may be more problematic. Nine of 40 treated patients (23%) had increases in alanine aminotransferase (ALT) at least 3 times the upper limit of the normal range. Median ALT levels were higher in patients in the higher dose groups. Four of 8 subjects in the 400 mg dose group discontinued treatment due to increases in ALT, prompting termination of dosing in this group. All abnormal ALT levels normalised after discontinuation of treatment.

While the study findings are limited by small patient numbers and the short duration of treatment, the lipid responses seen with mipomersen compare favourably with other treatments. Of note, in the higher dose groups, reductions in LDL cholesterol and apoB (>45%) exceeded levels observed with more potent statins. Mipomersen also effectively lowers Lp(a). A recent EAS Consensus Panel paper¹⁰ has identified elevated Lp(a) as a priority for treatment, after LDL cholesterol, in people at moderate to high cardiovascular risk. Currently, niacin is the only available treatment for elevated Lp(a). However, the frequency of elevations in liver enzymes may be cause for concern and justifies further evaluation. The authors conclude: ‘Further studies are needed to determine whether lowering of apoB by this novel mechanism may in fact predispose to liver transaminase elevations.’

Further genetic insights on hypertriglyceridemia susceptibility

The aetiological link between triglycerides and heart disease is an increasingly ‘hot topic’ in the literature. Genome-wide association studies (GWAS) have provided new insights into the regulation of plasma triglycerides, and whether triglycerides are causally related to coronary heart disease. This has been recently and comprehensively reviewed.¹¹

The genetics of hypertriglyceridemia, a defining characteristic of Fredrickson hyperlipidemia phenotypes, has yet to be fully elucidated, although previous studies suggest shared genetic aetiology. The current study provided a more comprehensive analysis of the genetics of the hyperlipoproteinemia-hypertriglyceridemia phenotypes based on data from 504 European patients and 1213 population-based controls.¹² The strength of this analysis is supported by the inclusion of all recently identified lipid-associated variants from the Global Lipids Genetics Consortium.^13,14

The study clearly showed that genetic loci for triglycerides, HDL cholesterol and LDL cholesterol associate with hypertriglyceridemia. The association was more robust for variants associated with triglycerides.

There was accumulation of common triglyceride-associated variants across the spectrum of hyperlipoproteinemia-hypertriglyceridemia phenotypes. In contrast, common variants associated with HDL cholesterol accumulate preferentially in types 4 and 5 phenotypes, whereas LDL cholesterol variants preferentially accumulate in types 2B and 5 phenotypes.

The study also showed rare variants associated with hypertriglyceridemia – including APOA5, GCKR, LPL and APOB – also accumulated across all phenotypes compared with controls.

Using a modelling approach, the authors showed that common variants explained 17.4% and rare variants 1.4% of the total variation in patients with hypertriglyceridemia. The authors suggested that the overall genetic burden of these variants might predispose patients to hypertriglyceridemia; however, there is no single variant(s) that causes hypertriglyceridemia or the hyperlipoproteinemia phenotypes. The findings support a polygenic model of hypertriglyceridemia.