Propranolol for probucol-induced QT prolongation with polymorphic ventricular tachycardia

Probucol aggravates long QT syndrome associated with a novel missense mutation M124T in the N-terminus of HERG

Patients with LQTS (long QT syndrome) with a mutation in a cardiac ion channel gene, leading to mild-to-moderate channel dysfunction, may manifest marked QT prolongation or torsade de pointes only upon an additional stressor. A 59-year-old woman had marked QT prolongation and repeated torsade de pointes 3 months after initiation of probucol, a cholesterol-lowering drug. We identified a single base substitution in the HERG gene by genetic analysis. This novel missense mutation is predicted to cause an amino acid substitution of Met(124)-->Thr (M124T) in the N-terminus. Three other relatives with this mutation also had QT prolongation and one of them had a prolonged QT interval and torsade de pointes accompanied by syncope after taking probucol. We expressed wild-type HERG and HERG with M124T in Xenopus oocytes and characterized the electrophysiological properties of these HERG channels and the action of probucol on the channels. Injection of the M124T mutant cRNA into Xenopus oocytes resulted in expression of functional channels with markedly smaller amplitude. In both HERG channels, probucol decreased the amplitude of the HERG tail current, decelerated the rate of channel activation, accelerated the rate of channel deactivation and shifted the reversal potential to a more positive value. The electrophysiological study indicated that QT lengthening and cardiac arrhythmia in the two present patients were due to inhibition of I(Kr) (rapidly activating delayed rectifier K(+) current) by probucol, in addition to the significant suppression of HERG current in HERG channels with the M124T mutation.

Torsade de pointes due to noncardiac drugs: most patients have easily identifiable risk factors

Numerous medications, including drugs prescribed for noncardiac indications, can lead to QT prolongation and trigger torsade de pointes. Although this complication occurs only rarely, it may have lethal consequences. It is therefore important to know if patients with torsade de pointes associated with noncardiac drugs have risk factors that are easy to identify. We reviewed reports of drug-induced torsade de pointes and analyzed each case of torsade de pointes associated with a noncardiac drug for the presence of risk factors for the long QT syndrome that can be easily identified from the medical history or clinical evaluation (female gender, heart disease, electrolyte disturbances, excessive dosing, drug interactions, and history of familial long QT syndrome). We identified 249 patients with torsade de pointes caused by noncardiac drugs. The most commonly identified risk factor was female gender (71%). Other risk factors were frequently present (18%-41%). Virtually all patients had at least 1 of these risk factors, and 71% of patients had 2 or more risk factors. Our study suggests that almost all patients with torsade de pointes secondary to noncardiac drugs have risk factors that can be easily identified from the medical history before the initiation of therapy with the culprit drug.

Clinical results with AGI-1067: a novel antioxidant vascular protectant

A large body of evidence points to oxidative stress as an important trigger in the complex chain of events leading to atherosclerosis. Reactive oxygen species have also been implicated in the pathophysiology of restenosis after percutaneous coronary interventions (PCI). The powerful antioxidant probucol has been shown to prevent coronary restenosis after balloon angioplasty in the MultiVitamins and Probucol (MVP) trial and other clinical studies. Probucol has also induced regression of carotid atherosclerosis in the Fukuoka Atherosclerosis Trial (FAST). However, prolongation of the QT interval with probucol remains a long-term safety concern. AGI-1067, a metabolically stable analog of probucol, is a vascular protectant (V-protectant) with strong antioxidant properties, equipotent to those of probucol. This V-protectant has been effective at preventing atherosclerosis in all tested animal models, including the low-density lipoprotein receptor-deficient and apolipoprotein E-knockout mice and the hypercholesterolemic primate. AGI-1067 improved luminal dimensions of the PCI site and reduced restenosis in the Canadian Antioxidant Restenosis Trial (CART-1). In contrast to probucol, AGI-1067 did not induce prolongation of the QT interval. AGI-1067 also improved luminal dimensions of the reference segments in the PCI vessels in CART-1, an effect that suggests a direct antiatherosclerosis effect. This has potentially important implications, as local approaches to prevent restenosis, such as coated stents, are not expected to prevent atherosclerosis progression, myocardial infarction, and cardiovascular death. Considering that oxidative stress and inflammation may persist for a prolonged period after stenting, treatment with AGI-1067 for the entire period of risk after PCI (instead of only 4 weeks in CART-1) may result in enhanced protection against luminal renarrowing in the ongoing multicenter CART-2 trial. Because the ultimate goal of therapy for patients with coronary artery disease must remain prevention of disease progression and atherosclerosis-related events, CART-2 will test the value of AGI-1067 for the reduction of both post-PCI restenosis and atherosclerosis progression.

Prevention of restenosis with antioxidants: mechanisms and implications

The aim of this review is to give an overview of the field of restenosis prevention with antioxidants, put in the perspective of their potential use for the prevention of atherosclerosis progression. Compelling evidence points to oxidative stress as an important trigger in the complex chain of events leading to atherosclerosis. There is also evidence that oxidative stress occurs early after angioplasty. Reactive oxygen species (ROS) can induce endothelial dysfunction and macrophage activation, resulting in the release of cytokines and growth factors that stimulate matrix remodeling and smooth muscle cell proliferation. The accumulation of new extracellular matrix and smooth muscle cells will result in the neointimal formation responsible for lumen narrowing after stent deployment and which contributes to that after balloon angioplasty. In addition, oxidation processes are involved in the cross-linking of collagen fibers, and this coupled with smooth muscle cell contraction and endothelial dysfunction may result in long-term vascular constriction or lack of adaptive vascular remodeling after balloon angioplasty. The powerful antioxidant probucol has been shown to prevent coronary restenosis after balloon angioplasty in the Multivitamins and Probucol (MVP) trial and other clinical studies. However, prolongation of the QT interval with probucol remains a long-term safety concern. AGI-1067, a metabolically stable analog of probucol, is a vascular protectant with strong antioxidant properties as potent to those of probucol. There has been no evidence of prolongation of the QT interval with AGI-1067 in initial clinical studies. The anti-restenosis properties of AGI-1067 are being assessed in the Canadian Antioxidant Restenosis Trial (CART)-1. Considering that oxidative stress and inflammation may persist for a prolonged period after stent placement, treatment with AGI-1067 for the entire period of risk after percutaneous coronary intervention (PCI) [instead of only 4 weeks in CART-1] may result in enhanced protection against luminal renarrowing. This hypothesis will be tested in the randomized, multicenter CART-2 trial. AGI-1067 has been effective at preventing atherosclerosis in all tested animal models, including the low density lipoprotein receptor-deficient and apo-E knockout mice. This has potentially important implications, as PCI and local approaches to prevent restenosis such as coated stents are not expected to prevent atherosclerosis progression, myocardial infarction and cardiovascular death. As the ultimate goal of therapy for patients with coronary artery disease must remain prevention of disease progression and atherosclerosis-related events, CART-2 will test the value of AGI-1067 for the reduction of both post-PCI restenosis and atherosclerosis progression.

Environmental estrogen stimulation of growth and estrogen receptor function in preneoplastic and cancerous human breast cell lines

BACKGROUND

DDT and polychlorinated biphenyls (PCBs), which are widespread in the ecosystem, can mimic estrogen-mediated cell activities. Thus, they can potentially interfere with many physiologic processes. We compared the effects of organochlorines belonging to the DDT and PCB families, alone and in combination, for their ability to influence the estrogen receptor-mediated activities in preneoplastic breast epithelial cells and breast cancer cells.

METHODS

Multiple assay systems requiring functional estrogen receptor were employed to test estrogen-like activity of organochlorine ligands. Two-sided statistical tests were used to compare the data.

RESULTS

p,p'-DDT, the predominant form of DDT in the environment, is a more potent estrogen than o,p'-DDT (P<.001), although it is less effective than o,p'-DDT in inhibiting the binding of estradiol (natural estrogen) to estrogen receptor. Among the PCBs, Heptachlor is estrogenic (in transient reporter assays; P< or =.001), whereas Aroclor 1221 and Aroclor 1254, both individually and in combination, are only weakly estrogenic.

CONCLUSION

p,p'-DDT is the most effective organochlorine in regulating estrogen receptor-mediated cellular responses. In estrogen receptor-positive breast cancer cells, p,p'-DDT evokes responses by itself and enhances the responses in collaboration with estradiol or o,p'-DDT.

Probucol-associated tachyarrhythmic events and QT prolongation: importance of gender

From published articles and adverse reactions reports filed with the FDA (available through the Freedom of Information Act), we analyzed occurrences of tachyarrhythmias and the magnitude of QTc prolongation associated with probucol therapy. Of 16 cases of tachyarrhythmic events reported in association with probucol, 15 (94%) occurred in women (p < 0.01 vs expected value of 58%). Tachyarrhythmias were specifically described as TdP in 11 (63%) cases, all women; additional potential contributory QT-prolonging factors (besides probucol) were not identifiable in 2 of the 11 cases. We also analyzed QTc responses in 359 probucol-treated patients, all having baseline QTc < or = 0.44 sec1/2. At doses of 500 to 1000 mg/day, probucol-associated prolongation of QTc to values > or = 0.45 sec1/2 was observed in 22% of women versus 7% of men (p < 0.001) and to values > or = 0.47 sec1/2 in 8% of women versus 2% of men (p < 0.03). Multivariate analysis identified baseline QTc (p < 0.0001) and female gender (p < 0.03), but neither age nor dose, as significant independent predictors of QTc prolongation to > or = 0.45 sec1/2 with probucol. These findings have relevance to the clinical use of probucol, provide further evidence that women have a relatively greater predisposition to development of acquired long QT syndrome, and carry implications for the design of trials involving QT-prolonging drugs.

The effect of probucol and vitamin E treatment on the oxidation of low-density lipoprotein and forearm vascular responses in humans

This study investigates the hypothesis that lipid soluble antioxidants may increase the resistance of low-density lipoprotein (LDL) to oxidation and also enhance vascular endothelial responses in humans. In a double-blind parallel group study, 24 hypercholesterolaemic patients already on treatment with simvastatin (20 mg day-1), were randomized to supplementary treatment with probucol (500 mg bd), vitamin E (400 IU daily) or placebo for 8 weeks. Mean serum cholesterol before antioxidant treatment was 7.00 mmol l-1. Resistance of LDL to oxidation by copper was increased by 830% in the probucol group and by 30% in the vitamin E group. However, thiobarbituric acid reacting substances in whole serum were not altered by either antioxidant. Probucol lowered HDL- and LDL-cholesterol levels and increased the QT interval. Forearm vascular responses, as measured by venous occlusion plethysmography, to acetylcholine, glyceryl trinitrate and NG-monomethyl-L-arginine, were not significantly changed by antioxidant treatment. Probucol has a major, and vitamin E a minor, effect on LDL resistance to oxidation but neither compound appears to alter forearm vascular responses in vivo.