Cimetidine, sold under the brand name Tagamet among others, is a histamine H2 receptor antagonist that inhibits stomach acid production.[6][7][8] It is mainly used in the treatment of heartburn and peptic ulcers.[6][8][9]
With the development of proton pump inhibitors, such as omeprazole, approved for the same indications, cimetidine is available as an over-the-counter formulation to prevent heartburn or acid indigestion, along with the other H2-receptor antagonists.[10]
Cimetidine was developed in 1971 and came into commercial use in 1977.[11][12] Cimetidine was approved in the United Kingdom in 1976, and was approved in the United States by the Food and Drug Administration in 1979.[13]
Cimetidine is indicated for the treatment of duodenal ulcers, gastric ulcers, gastroesophageal reflux disease, and pathological hypersecretory conditions.[1] Cimetidine is also used to relieve or prevent heartburn.[2]
Reported side effects of cimetidine include diarrhea, rashes, dizziness, fatigue, constipation, and muscle pain, all of which are usually mild and transient.[14] It has been reported that mental confusion may occur in the elderly.[14] Because of its hormonal effects, cimetidine rarely may cause sexual dysfunction including loss of libido and erectile dysfunction and gynecomastia (0.1–0.2%) in males during long-term treatment.[14][15][16] Rarely, interstitial nephritis, urticaria, and angioedema have been reported with cimetidine treatment.[14] Cimetidine is also commonly associated with transient raised aminotransferase activity; hepatotoxicity is rare.
Cimetidine appears to be very safe in overdose, producing no symptoms even with massive overdoses (e.g., 20 g).
Due to its non-selective inhibition of cytochrome P450 enzymes, cimetidine has numerous drug interactions. Examples of specific interactions include, but are not limited to, the following:
The mechanism of action of cimetidine as an antacid is as a histamine H2 receptor antagonist.[25] It has been found to bind to the H2 receptor with a Kd of 42 nM.[26]
Cimetidine is a potent inhibitor of certain cytochrome P450 (CYP) enzymes,[27][28] including CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4.[27][28][29] The drug appears to primarily inhibit CYP1A2, CYP2D6, and CYP3A4,[30] of which it is described as a moderate inhibitor.[31] This is notable since these three CYP isoenzymes are involved in CYP-mediated drug biotransformations;[32] however, CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 are also involved in the oxidative metabolism of many commonly used drugs.[33] As a result, cimetidine has the potential for a large number of pharmacokinetic interactions.[27][28][29]
Cimetidine is reported to be a competitive and reversible inhibitor of several CYP enzymes,[34][35][28][36] although mechanism-based (suicide) irreversible inhibition has also been identified for cimetidine's inhibition of CYP2D6.[37] It reversibly inhibits CYP enzymes by binding directly with the complexed heme-iron of the active site via one of its imidazole ring nitrogen atoms, thereby blocking the oxidation of other drugs.[28][36]
Cimetidine has been found to possess weak antiandrogenic activity at high doses.[25][39][40][41] It directly and competitively antagonizes the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT).[42][43] However, the affinity of cimetidine for the AR is very weak; in one study, it showed only 0.00084% of the affinity of the anabolic steroid
The mechanism of action of cimetidine as an antacid is as a histamine H2 receptor antagonist.[25] It has been found to bind to the H2 receptor with a Kd of 42 nM.[26]
Cimetidine is a potent inhibitor of certain cytochrome P450 (CYP) enzymes,[27][28] including CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4.[27][28][29] The drug appears to primarily inhibit CYP1A2, CYP2D6, and CYP3A4,[30] of which it is described as a moderate inhibitor.[31]
Cimetidine has been found to possess weak antiandrogenic activity at high doses.[25][39][40][41] It directly and competitively antagonizes the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT).[42][43] However, the affinity of cimetidine for the AR is very weak; in one study, it showed only 0.00084% of the affinity of the anabolic steroid metribolone
Cimetidine is rapidly absorbed regardless of route of administration.[4] The oral bioavailability of cimetidine is 60 to 70%.[3] The onset of action of cimetidine when taken orally is 30 minutes,[31] and peak levels occur within 1 to 3 hours.[3] Cimetidine is widely distributed throughout all tissues.[4] It is able to cross the blood–brain barrier and can produce effects in the central nervous system (e.g., headaches, dizziness, somnolence).[5]
Cimetidine, approved by the FDA for inhibition of gastric acid secretion, has been advocated for a number of dermatological diseases.[58] Cimetidine was the prototypical histamine H2 receptor antagonist from which the later members of the class were developed. Cimetidine was the culmination of a project at Smith, Kline & French (SKF) Laboratories in Welwyn Garden City (now part of GlaxoSmithKline) by James W. Black, C. Robin Ganellin, and others to develop a histamine receptor antagonist to suppress stomach acid secretion.[59] This was one of the first drugs discovered using a rational drug design approach. Sir James W. Black shared the 1988 Nobel Prize in Physiology or Medicine for the discovery of propranolol and also is credited for the discovery of cimetidine.
At the time (1964), histamine was known to stimulate the secretion of stomach acid, but also that traditional antihistamines had no effect on acid production. In the process, the SK&F scientists also proved the existence of histamine H2 receptors.
The SK&F team used a rational drug-design structure starting from the structure of histamine — the only design lead, since nothing was known of the then hypothetical H2 receptor. Hundreds of modified compounds were synthesized in an effort to develop a model of the receptor. The first breakthrough was Nα-guanylhistamine, a partial H2 receptor antagonist. From this lead, the receptor model was further refined and eventually led to the development of burimamide, the first H2 receptor antagonist. Burimamide, a specific competitive antagonist at the H2 receptor, 100 times more potent than Nα-guanylhistamine, proved the existence of the H2 receptor.
Burimamide was still insufficiently potent for oral administration, and further modification of the structure, based on modifying the pKa of the compound, led to the development of metiamide. Metiamide was an effective agent; it was associated, however, with unacceptable nephrotoxicity and agranulocytosis.[59] The toxicity was proposed to arise from the thiourea group, and similar guanidine analogues were investigated until the ultimate discovery of cimetidine. The compound was synthesized in 1972 and evaluated for toxicology by 1973. It passed all trials.
Cimetidine was first marketed in the United Kingdom in 1976, and in the U.S. in August 1977; therefore, it took 12 years from initiation of the H2 receptor antagonist program to commercialization. By 1979, Tagamet was being sold in more than 100 countries and became the top-selling prescription product in the U.S., Canada, and several other countries. In November 1997, the American Chemical Society and the Royal Society of Chemistry in the U.K. jointly recognized the work as a milestone in drug discovery by designating it an International Historic Chemical Landmark during a ceremony at SmithKline Beecham's New Frontiers Science Park research facilities in Harlow, England.[60]
The commercial name "Tagamet" was decided upon by fusing the two words "antagonist" and "cimetidine".[59] Subsequent to the introduction onto the U.S. drug market, two other H2 receptor antagonists were approved, ranitidine (Zantac, Glaxo Labs) and famotidine (Pepcid, Yamanouchi, Ltd.) Cimetidine became the first drug ever to reach more than $1 billion a year in sales, thus making it the first blockbuster drug.[61]
Tagamet has been largely replaced by proton pump inhibitors for treating peptic ulcers, but is available as an over-the-counter medicine for heartburn in many countries.[60]
On 21 August 1989, Danlex Research Laboratories, Inc. filed a petition with the Filipino Bureau of Patents, Trademarks and Technology Transfer (BPTTT) for a compulsory license to manufacture, use, and sell Cimetidine, citing the provisions of the Patent Law (R.A. 165) for medicinal products. Pre-1995, the Bureau of Patents, Trademarks and Technology Transfer (BPTTT) ruled in favor of Danlex and granted the compulsory license, subject to Danlex paying a royalty of 2.5% of net sales to Smith, Kline & French. On 27 January 1995, Smith, Kline & French appealed to the Court of Appeals, but the CA affirmed the BPTTT's decision, upholding the grant of the compulsory license to Danlex. On 25 July 1995, the Court of Appeals denied Smith, Kline & French's Motion for Reconsideration. On 15 September 1995, Smith, Kline & French filed a Petition for Review on Certiorari with the Supreme Court of the Philippines, challenging the CA's decision. On 23 October 2001, the Supreme Court (G.R. No. 121267) rendered its decision, affirming the Court of Appeals and upholding the grant of the compulsory license to Danlex. This marked the final judicial resolution of the case.[62]
Some evidence suggests cimetidine could be effective in the treatment of common warts, but more rigorous double-blind clinical trials found it to be no more effective than a placebo.[63][64][65]
Tentative evidence supports a beneficial role as add-on therapy in colorectal cancer.[66]
Cimetidine inhibits ALA synthase activity and hence may have some therapeutic value in preventing and treating acute porphyria attacks.[67][68]
There is some evidence supporting the use of cimetidine in the treatment of PFAPA.[69]
In dogs, cimetidine is used as an antiemetic when treating chronic gastritis.[70]
At typical therapeutic levels, cimetidine has either no effect on or causes small increases in circulating testosterone concentrations in men. Any increases in testosterone levels with cimetidine have been attributed to the loss of negative feedback on the HPG axis that results due to AR antagonism.[45] At typical clinical dosages, such as those used to treat peptic ulcer disease, the incidence of gynecomastia (breast development) with cimetidine is very low at less than 1%.[52][53] In one survey of over 9,000 patients taking cimetidine, gynecomastia was the most frequent endocrine-related complaint but was reported in only 0.2% of patients.[53] At high doses however, such as those used to treat Zollinger–Ellison syndrome, there may be a higher incidence of gynecomastia with cimetidine.[52] In one small study, a 20% incidence of gynecomastia was observed in 25 male patients with duodenal ulcers who were treated with 1,600 mg/day cimetidine.[51] The symptoms appeared after 4 months of treatment and regressed within a month following discontinuation of cimetidine.[51] In another small study, cimetidine was reported to have induced breast changes and erectile dysfunction in 60% of 22 men treated with it.[51] These adverse effects completely resolved in all cases when the men were switched from cimetidine to ranitidine.[51] A study of the United Kingdom General Practice Research Database, which contains over 80,000 men, found that the relative risk of gynecomastia in cimetidine users was 7.2 relative to non-users.[51] People taking a dosage of cimetidine of greater than or equal to 1,000 mg showed more than 40 times the risk of gynecomastia than non-users.[51] The risk was highest during the period of time of 7 to 12 months after starting cimetidine.[51] The gynecomastia associated with cimetidine is thought to be due to blockade of ARs in the breasts, which results in estrogen action unopposed by androgens in this tissue, although increased levels of estrogens due to inhibition of estrogen metabolism is another possible mechanism.[51] Cimetidine has also been associated with oligospermia (decreased sperm count) and sexual dysfunction (e.g., decreased libido, erectile dysfunction) in men in some research, which are hormonally related similarly.[45][53][51]
In accordance with the very weak nature of its AR antagonistic activity, cimetidine has shown minimal effectiveness in the treatment of androgen-dependent conditions such as acne, hirsutism (excessive hair growth), and hyperandrogenism (high androgen levels) in women.[54][55][52][56] As such, its use for such indications is not recommended.[55][56]
Cimetidine is reported to be a competitive and reversible inhibitor of several CYP enzymes,[34][35][28][36] although mechanism-based (suicide) irreversible inhibition has also been identified for cimetidine's inhibition of CYP2D6.[37] It reversibly inhibits CYP enzymes by binding directly with the complexed heme-iron of the active site via one of its imidazole ring nitrogen atoms, thereby blocking the oxidation of other drugs.[28][36][38]
At typical therapeutic levels, cimetidine has either no effect on or causes small increases in circulating testosterone concentrations in men. Any increases in testosterone levels with cimetidine have been attributed to the loss of negative feedback on the HPG axis that results due to AR antagonism.[45] At typical clinical dosages, such as those used to treat peptic ulcer disease, the incidence of gynecomastia (breast development) with cimetidine is very low at less than 1%.[52][53] In one survey of over 9,000 patients taking cimetidine, gynecomastia was the most frequent endocrine-related complaint but was reported in only 0.2% of patients.[53] At high doses however, such as those used to treat Zollinger–Ellison syndrome, there may be a higher incidence of gynecomastia with cimetidine.[52] In one small study, a 20% incidence of gynecomastia was observed in 25 male patients with duodenal ulcers who were treated with 1,600 mg/day cimetidine.[51] The symptoms appeared after 4 months of treatment and regressed within a month following discontinuation of cimetidine.[51] In another small study, cimetidine was reported to have induced breast changes and erectile dysfunction in 60% of 22 men treated with it.[51] These adverse effects completely resolved in all cases when the men were switched from cimetidine to ranitidine.[51] A study of the United Kingdom General Practice Research Database, which contains over 80,000 men, found that the relative risk of gynecomastia in cimetidine users was 7.2 relative to non-users.[51] People taking a dosage of cimetidine of greater than or equal to 1,000 mg showed more than 40 times the risk of gynecomastia than non-users.[51] The risk was highest during the period of time of 7 to 12 months after starting cimetidine.[51] The gynecomastia associated with cimetidine is thought to be due to blockade of ARs in the breasts, which results in estrogen action unopposed by androgens in this tissue, although increased levels of estrogens due to inhibition of estrogen metabolism is another possible mechanism.[51] Cimetidine has also been associated with oligospermia (decreased sperm count) and sexual dysfunction (e.g., decreased libido, erectile dysfunction) in men in some research, which are hormonally related similarly.[45][53][51]
In accordance with the very weak nature of its AR antagonistic activity, cimetidine has shown minimal effectiveness in the treatment of androgen-dependent conditions such as acne, hirsutism (excessive hair growth), and hyperandrogenism (high androgen levels) in women.[54][55][52][56] As such, its use for such indications is not recommended.[55][56]
