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HMG COA reductase inhibitors - Statins

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HMG CoA reductase inhibitors competitively inhibit the activity of HMG CoA reductase, the rate-limiting enzyme in cholesterol synthesis. Inhibition of this enzyme results in a transient, modest decrease in cellular cholesterol concentration . The decrease in cholesterol concentration activates a cellular signaling cascade culminating in the activation of sterol regulatory element binding protein (SREBP), a transcription factor that up-regulates expression of the gene encoding the LDL receptor. Increased LDL receptor expression causes increased uptake of plasma LDL, and consequently decreases plasma LDL-cholesterol concentration. Approximately 70% of LDL receptors are expressed by hepatocytes, with the remainder expressed by a variety of cell types in the body.

HMG CoA reductase inhibitors,or statins are widely prescribed drugs. They are indicated for the treatment of hipercholesterolemia, a condition that increases cardiovascular risk. Currently, the drugs of this group that are available are:

Hmg Coa Reductase Inhibitor Pictures

微纖維酸類(Fibrates)
例子:Benzafibrate,Ciprofibrate,Fenofibrate,Gemfibrozil。非常有效地減低三酯甘油,但對於降LDL-C的效果就不是太理想。常見的副作用有輕微腸胃不適、頭暈、皮膚出疹。此類藥物應避免用於腎功能差和有甲狀腺功能減退的病人;而且,在與他汀類共同使用的時候,要特別留意由此引起的肌肉病變。如果醫生處方每日劑量分兩次服用,病人可在早餐前和晚餐前30分鐘服用。
膽酸結合劑(BileAcidSequestrants)
例子:Cholestyramine,Colesevelam,Colestipol。膽酸結合劑(BileAcidSequestrants)能和膽汁結合,有效防止LDL-C重新吸收和提高LDL-C排出體外,但它亦有可能會增加體內的三酯甘油。由於膽酸結合劑並不能在腸內被吸收,最常見的副作用是腸胃不適和便秘,不過它們亦可能引起腹瀉。此外,膽酸結合劑會阻礙油溶性維他命的吸收,病人可能會出現維他命A、D和K缺乏而產生的症狀,例如由於缺乏維他命K引起的容易出血。另外,此類藥物會減低其他藥物的吸收,所以如無特別指示,膽酸結合劑和其他藥物應最少分開一小時服用。
膽固醇吸收抑制劑
例子:Ezetimibe,  Ezetimibe是最新的降膽固醇藥並已獲美國食物及藥品管理局(FDA)註冊作為單一治療或可共他汀類一起使用。它的主要作用是能減低腸臟對食物膽固醇的吸收。其常見的副作用有腹瀉、腹痛、頭痛等。
HMG CoA Reductase Inhibitors (Statins)
是目前最有效的膽固醇藥。此藥主要的作用是降低低密度膽固醇值,其特點是效度高,當使用一般治療劑量時,可以降低低密度膽固醇值20~40%。使用方便又副作用少,所以病患的接受度高。此藥是抑制膽固醇合成的速率決定步驟的酶,是直接抑制膽固醇生物合成關鍵酶的藥物,當然是最有效了。在治療劑量時,其可減少膽固醇的內生合成,但不會完全抑制。因而使肝細胞增加低密度膽固醇接受體的表現,進一步促進血漿中低密度膽固醇的移除。此作用機轉使其降膽固醇效果卓著。
煙酸類(NicotinicAcid/Niacin)
例子:Acipimox及NicotinicAcid,煙酸類(NicotinicAcid)不但能減低LDL-C和三酯甘油,更可以提高血液內HCL-C,對治療高血脂症非常有效。不過,大部分病人服用煙酸類藥物都會出現難以忍受的臉部通紅、皮膚痕癢和腸胃不適等副作用。現在煙酸已經不常作降膽固醇之用。此外,煙酸亦不宜用於有冠心病、糖尿病、高尿酸症和有腸胃潰瘍的病人。煙酸應該在飯時/飯後服用以減低其對腸胃產生的副作用。現在有藥廠生產控釋(ControlledRelease)劑型以減少煙酸對腸胃的影響,但是我們要注意,即使是同樣的主要成分,不同的劑型亦會令所需的劑量改變。
 

HMG CoA reductase inhibitors table pneumonics

Binding causes the LDL to be taken up by the liver cell and digested in a lysosome. Fatty acids and amino acids in the LDL are recycled, and the cholesterol enters the cholesterol pool in the liver cell. Note that if the liver cell synthesizes cholesterol, this too will be added to the cholesterol pool. If the concentration of cholesterol in the liver cells exceeds a certain level, HMG CoA reductase is inhibited, and the synthesis of LDL receptors is also inhibited.