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Statins for Cancer

This is a VERY LARGE topic to review (just like “cox-2 inhibitors” and “PPAR gamma agonists” because of the abundant data and complexity of the background biology – which is why I haven’t gotten to it sooner, sorry !).

Statins is the nick name for so-called HMG-CoA reductase inhibitors and well established for treating high cholesterol. Statin research started with Japanese researchers Endo and Kuroda in 1971 and within a decade Merck has launched lovastatin (Mevacor), the first statin drug.  Today, statins are the second most common class of drugs prescribed in the US after analgesics, and are commonly used for hyperlipidemia and to reduce the risk of heart attack and stroke. What makes statins intriguing is that these drugs have other biological actions beyond the lowering of cholesterol which makes them potentially useful in a bewildering array of medical conditions such as auto-immune conditions, organ transplantation, polycystic ovarian syndrome (PCOS), arrhythmias, chronic obstructive pulmonary disease (COPD), sepsis, contrast-induced nephropathy, cataract, age-related macular degeneration, sub-arachnoid hemorrhage, osteoporosis, dementia, asthma, thromboembolism, Alzheimer and even cancer.  As a class, statins have diverse biological effects including improving endothelial function, stabilizing atherosclerotic plaques, attenuating oxidative stress and inflammation, immuno-modulation, as well as inhibition the thrombogenic response. Many of these actions are executed via a modulation or inhibition of post-translational protein prenylation (also called isoprenylation). The basic biology behind statin effects is way too large a topic to review here and I would suggest reading articles like “Statins: a new insight into their mechanisms of action and consequent pleiotropic effects” in Pharmacol Rep. 2007 Sep-Oct;59(5):483-99 or the excellent downloadable article in Nature Reviews Immunology “Statin therapy and autoimmune disease: from protein prenylation to immunomodulation” for further insight. For our purposes, it suffices to know that the immunomodulatory, antiangiogenic and anti-inflammatory effects of statins all contribute to its anti-cancer potential (for an overview of “statins in tumour suppression”, also see Sassano and Platanias’s article of this title in Cancer Letters 2008 Feb 18; 260(1-2):11-9), and instead we will focus on the practical applicability of the scientific findings to date.  So, here is the background:

a) In Vitro (cellular evidence):

There is a large body of in vitro evidence demonstrating the potential of statin’s use against cancer, especially of its potential synergies with other agents against cancer.  We will not be able to list them all, but will cite a sample of some recent research in various cancer types to illustrate as follows:

Breast Cancer – Many studies demonstrated in vitro inhibition of breast cancer cells by statins and synergistic effects with other agents as well, an example would be Campbell et al’s Breast cancer growth prevention by statins in Cancer Res., 2006 Sep 1;66(17):8707-14.

Lung Cancer – Inhibition of various lung cancer cell lines have been widely reported (See Maksimova, E. Lung. 2008 Jan-Feb;186(1):45-54) including against small cell lung cancer (Khanzada UK, et al. Oncogene. 2006 Feb 9;25(6):877-87). Moreover, Furthermore, synergism with greentea, cox-2 inhibitors, gamma tocotrienol, chemotherapy, bisphosphonates has been demonstrated in vitro.

Brain Cancer – Statin treatment inhibited proliferation and migration in human U251 and U87 glioma cells in a dose dependent manner.  Statins also induced apoptosis of glioma cells thought to be related to inhibition of the prosurvival PI3K/Akt pathway triggering caspase-3-dependent cell death through the modulation of lipid rafts (Wu H, et al. Neurosurgery. 2009 Dec;65(6):1087-96). Earlier in 2004, it was also reported that simvastatin induced proliferation inhibition and apoptosis in C6 glioma cells via c-jun N-terminal kinase (Neurosci Lett. 2004 Nov 11;370(2-3):212-7).

Colon Cancer – There are many reports of statins inducing apoptosis in human colon cancer cells and preventing its carcinogenesis. Furthermore, synergism with cox-2 inhibitors, gamma tocotrienol, chemotherapy, TRAIL, butyrate has been demonstrated in vitro and studies showing statins ability to reduce inflammation in colon tissue raises the hope that it may be useful in inflammatory bowel diseases as a chemopreventative against colon cancer.

Ovarian Cancer – There is research to establish that statins induces cell death in ovarian cancer. What is interesting is the existence of possible differential effect between lipophilic (eg simvastatin) v. hydrophilic (e.g. pravastatin) statins (Kato et al, J Cell Mol Med. 2009 May 11)

Prostate Cancer – There are many reports of statins inducing apoptosis in prostate cancer (e.g. PC-3) cells via various mechanisms (Hoque A, et al. Cancer Epidemiol Biomarkers Prev. 2008 Jan;17(1):88-94), e.g. Murtola et al’s recent results demonstrating simvastatin to inhibit prostate epithelial cell growth at clinically relevant doses (Prostate. 2009 Jun 15;69(9):1017-23). Furthermore, synergism with cox-2 inhibitors, PPAR agonists, bisphosphonates, HDAC inhibitors, tocotrienols in vitro have all been reported.

Pancreas Cancer – Statins reduce pancreatic cancer growth, invasion and metastasis (Kusama et al. Gastroenterology. 2002 Feb;122(2):308-17). Mistafa and Stenius from the Karolinska Institute in Sweden recently reported that atorvastatin (Lipitor) decreased constitutive- and insulin-induced pAkt in Panc-1 and MIA PaCa-2 cells and also inhibited pAkt in combination with gemcitabine and 5-fluorouracil chemotherapy, and sensitized cells to gemcitabine and 5-fluorouracil induced apoptosis and inhibition of cell proliferation (Biochem Pharmacol. 2009 Nov 1;78(9):1115-2). Czech researchers comparing various statins against pancreas cancer in vitro identified also simvastatin as the most potent (Int J Cancer. 2008 Mar 15;122(6):1214-21)

Comparable research showing the usefulness of statins against cancer cell lines have been published against a wide range of other cancers (liver, sarcoma, leukemia, lymphoma), and as noted above, synergies in vitro with cox inhibitors, green tea polyphenols, HDAC inhibitors, PPAR agonists, gamma tocotrienols, chemotherapeutic agents, bisphosphonates, and even oncolytic viruses etc. have also been documented (also see below section on synergisms).

In Vivo (animal evidence):

There is a similarly large body of evidence that statins are bioactive as anti-cancer agents both in the prevention and suppression of cancer in laboratory animals, and a sampling of such research is as follows:

Cancer Prevention

A new lipophilic statin pitavastatin attenuates chronic inflammation and improves the imbalance of adipocytokines, thus preventing the development of colonic premalignancies in C57BL/KsJ-db/db (db/db) obese mice (Cancer Sci. 2010 Mar 24).

Atorvastatin inhibits intestinal tumorigenesis especially when given together with low doses of celecoxib in APC(min) rodent model (Cancer Res. 2006 Jul 15;66(14):7370-7)

Studies in severe combined immunodeficiency (SICD) mice show that simvastatin delays the development of EBV-lymphomas in these animals (Br J Cancer. 2005 May 9;92(9):1593-8).

Cancer Treatment

Low-dose simvastatin increased necrosis and apoptosis in an orthotopic mouse glioblastoma (GL-26) model (Anticancer Res. 2009 Dec;29(12):4901-8.).

Combination of atorvastatin and celecoxib prevented prostate cancer progression from androgen dependence to androgen independence in a mouse model (Cancer Prev Res (Phila Pa). 2010 Jan;3(1):114-24).

Simvastatin exhibited impairment of xenograft K562 chronic myelogenous leukemia (CML) cell growth in nude mice and also blocked cell cycle in G(1) phase (Chemotherapy. 2008;54(6):438-46).

Lovastatin inihibted tumor growth and lung metastasis in a mouse model of metastatic mammary cancer with a p53 mutation demonstrated by inoculating syngeneic BALB/c mice with BJMC3879 cells (Carcinogenesis. 2004 Oct;25(10):1887-98)

Clinical (human evidence):

Unlike the unequivocal in vitro and animal studies demonstrating clear-cut benefit of statins against a wide array of cancers, there is relative few data from the clinical arena.  In the early 2000s, there was actually a concern that statin use may be associated with an increased risk of cancer, but analyses of several large statin studies in cardiovascular diseases dispelled the concern. Human studies can be generally divided into the use of statins as a chemopreventative to prevent cancer or as an adjuvant to treat actual cancer. While a lot of effort in terms of direct studies and meta-analyses have been devoted to finding a correlation of statin use and cancer incidences, and while risk reductions of 48% – 90% were found for breast, colon and prostate cancers in retrospective case-control studies, these are by no means a final reliable statistic.

Breast Cancer – In a review by Kochhar and team of 40,421 females,  statin use was associated with a 51% risk reduction of breast cancer after controlling for age, smoking, alcohol use, and diabetes (J Clin Oncol 23: 7S, 2005 [suppl, abstr 514]). In the same vein, Kwan et al. observed that breast cancer patients who took statins after diagnosis were less likely to have had recurrences than were patients who did not take stains (Breast Ca Res Treat. 2008 Jun;109(3):573-9).  In 2008, Kumar et al. made the interesting observation that women on statins who develop breast cancers develop less aggressive cancers which are of lower grade and less invasive (Cancer Epidemiol Biomarkers Prev. 2008 May;17(5):1028-33.). Subsequently, Garwood et al. from UC San Francisco performed an interesting perioperative window trial of fluvastatin in 40 women with a diagnosis of DCIS or stage 1 breast cancer. Patients were randomized to high dose (80 mg/day) or low dose (20 mg/day) fluvastatin for 3-6 weeks before surgery and the research team found that the short treatment caused measurable favorable biologic changes by reducing tumor proliferation in high-grade, stage 0/1 breast cancer but not DCIS (Breast Ca Res Treat. 2010 Jan;119(1):137-44).

Colorectal Cancer – Researchers from the University of Michigan, collaborating with researchers in Israel, compared the use of statins among 1,953 patients with colorectal cancer and 2,015 other people who did not have the disease. This study specifically associated a 47 percent reduction in the risk of colorectal cancer with statin use (Poynter, JN., et al. New England J Med,. May 26, 2005, 352:2184–92)

Hepatocarcinoma –  A first study showing risk reduction of liver cancer from statin use was reported by El-Serag et al. from Baylor in 2009 where the risk reduction observed with statin use ranged was as high as 40% (Gastroenterology. 2009 May;136(5):1601-8). And as far as statins as an adjuvant therapy for liver cancer, Kawata et al. from Osaka reported their controlled trial in 91 patients with unresectable hepatocarcinoma randomly assigned to standard care v. standard care plus 40mg of daily pravastatin.  The result revealed that survival was doubled ( 18 v. 9 mos) for patients receiving pravastatin (Br J Cancer. 2001 Apr 6;84(7):886-91)

Graf et al. from Germany 183 HCC patients who had been selected for palliative treatment by transarterial chemoembolization (TACE). Fifty-two patients received TACE combined with pravastatin (20-40 mg/day) and 131 patients received chemoembolization alone. Six independent predictors of survival according to the Vienna survival model for HCC were equally distributed in both groups. RESULTS: During the observation period of up to 5 years, 31 (23.7%) out of 131 patients treated by TACE alone and 19 (36.5%) out of 52 patients treated by TACE and pravastatin survived. Median survival was significantly longer in HCC patients treated by TACE and pravastatin (20.9 months, 95% CI 15.5-26.3, p = 0.003) than in HCC patients treated by TACE alone (12.0 months, 95% CI 10.3-13.7) (Digestion. 2008;78(1):34-8)

Lung Cancer – Use of statins for more than 6 months reduced the risk for lung cancer by 55%, according to the results of a large scale case-control study involving almost a half million patients, published in the May 2007 issue of Chest. (Chest, 2007;131:1282-1288). What was intriguing was that longer duration of statin use was associated with enhanced risk reduction, rising from 55% to 77% for those who have been on statins for 4 years or more.

Prostate Cancer – Until only several years ago, only a handful of observational studies found statin use to be associated with reduced prostate cancer risk, though others found no association. By 2008 however, four large prospective studies (e.g. the California Men’s Health Study) have since observed similar reductions in the risk of advanced prostate cancer, although there was no reduction in the risk of overall prostate cancer.  For example, a meta-analysis of 19 studies [6 randomized clinical trials (RCTs), 6 cohort and 7 case-control studies] found that while long-term statin use did not seem to contribute to a reduced incidence of prostate cancer, statins did seemed to confer a protective effect in advanced prostate cancer (RR = 0.77, 95% CI: 0.64-0.93) (Int J Cancer. 2008 Aug 15;123(4):899-904).

As for statin use in patients who have had prostate cancer, a study examining statin use in close to 1000 patients with localized prostate cancer treated with brachytherapy found patients on statins to have lower PSAs and % positive biopsies than controls (Urol Nurs. 2006 Aug;26(4):298-303). The results were corroborated by Zelefsky et al’s recent report from Memorial Sloan-Kettering Cancer Center that high risk prostate cancer patients who have undergone radiotherapy had an improved PSA-relapse free survival and concluded that “data suggest that statins have anticancer activity and possibly provide radiosensitization when used in conjunction with radiotherapy in the treatment of prostate cancer” (Int J Radiat Oncol Biol Phys. 2010 May 6).  A meta-analysis of 19 studies [6 randomized clinical trials (RCTs), 6 cohort and 7 case-control studies] found that while long-term statin use did not seem to contribute to a reduced incidence of prostate cancer, however statins seemed to confer a protective effect in advanced prostate cancer (RR = 0.77, 95% CI: 0.64-0.93) (Int J Cancer. 2008 Aug 15;123(4):899-904). And most recently, a retrospecitive report on 23,320 patients in the Finnish prostate cancer screening trial found lower prostate cancer incidence in statin users (Int J Cancer. 2010 Jan 13)

And the latest study:  in the upcoming Jul 15th 2010 issue of Cancer, data from Duke by Stephen Freedland’s team show that in a series of 1300+ men who had prostatectomy for prostate cancer, there is a 30% lowered chance of recurrence for those patients who took statins.  Intriguingly, men who took the highest doses saw their recurrence risk drop 50%. In 300 men with biochemical recurrences during the follow-up,  16% were on statins vs. 25% who were not.

Myeloma – In a small European phase 2 study involving six myeloma patients refractory to two cycles of bortezomib or bendamustine, simvastatin was concomitantly administered during further cycles. Observation showed reduction of drug resistance by simvastatin (Eur J Haematol. 2007 Sep;79(3):240-3).

Potential synergies with other anti-cancer agents

This is a particularly interesting area. Perhaps because of the pleiotropic effects of statins themselves, it may influence the efficacy of many other anti-cancer or potential anti-cancer agents, either via biological synergism or by affecting the blood levels of some agents.  Although most of the studies represent in vitro work, such insights set a potential foundation for rational planning of a cocktailed approach against cancer.  Some representative research illustrating statin synergy potentials are as follows:

– potentiates sorafenib (Nexavar) cytotoxicity (Cancer Lett. 2010 Feb 1;288(1):57-67)

– synergizes with  , the cox-2 (Celebrex) inhibitor, against colon cancer (Int J Oncol. 2009 Nov;35(5):1037-43; also Int J Cancer. 2010 Feb 15;126(4):852-63)

– synergizes with sulindac (NSAID) against colon cancer (Gastroenterology. 1999 Oct;117(4):838-47)

– synergizes with gamma tocotrienols against breast cancer (Exp Biol Med (Maywood). 2009 Jun;234(6):639-50)

– enhances the all-trans retinoic acid (ATRA)-dependent antileukemic response in acute promyelocytic leukemia (Mol Cancer Ther. 2009 Mar;8(3):615-25)

– synergistic inhibition of lung tumorigenesis with green tea polyphenols (Clin Cancer Res. 2008 Aug 1;14(15):4981-8)

– potentiation of the effects of lenalidomide against myeloma (Leuk Res. 2009 Jan;33(1):100-8), and thalidomide as well (Eur J Clin Pharmacol. 2006 Apr;62(4):325-9)

– synergistic with an m-TOR inhiitor against acute leukemia (Anticancer Drugs. 2008 Aug;19(7):705-12)

– enhances the cytotoxic and apoptotic effects of doxorubicin chemotherapy against human colon cancer cells and in murine tumor models (Oncol Rep. 2008 May;19(5):1205-11)

– enhances the antiproliferative effects of gemcitabine chemotherapy against pancreas cancer in vitro (Br J Cancer. 2005 Aug 8;93(3):319-30)

– synergism with paclitaxel (Taxol) chemotherapy in K52 and HL60 cell lines (Mol Cancer Ther. 2001 Dec;1(2):141-9)

– potentiates cisplatinum against MmB16 melanoma in rodent model (Gastroenterology. 1999 Oct;117(4):838-47)

– potentiation anti-tumor effects of pamidronate (bisphosphonate) in vitro and in vivo (Int J Oncol. 2007 Jun;30(6):1413-25)

– synergizes with zoledronic acid (Zometa, a bisphosphonate) against myeloma (Anticancer Drugs. 2006 Jul;17(6):621-9)

– enhances trastuzumab (Herceptin) in breast cancer cell lines (Breast Cancer Res Treat. 2007 Jul;104(1):93-101)

– potentiates the effect of saquinavir against Daudi and Raji human lymphoma cells (Oncol Rep. 2004 Dec;12(6):1371-5)

– synergism with tamoxifen (Cardiovasc Res. 2004 Nov 1;64(2):346-55)

– synergistic effect with troglitazone (PPAR agonist) in majority of cell lines tested including DBTRG 05 MG (glioblastoma) and CL1-0 (lung) (Int J Cancer. 2006 Feb 1;118(3):773-9).

– synergizes with HDAC inhibitors against mice Lewis lung cancer model (Int J Cancer. 2002 Feb 20;97(6):746-50); and with phenylacetate against human glioma cells (J Neurochem. 1996 Feb;66(2):710-6).

– potetiation of photocytotoxic effect of photofrin II (Bull Acad Natl Med. 1994 Jun;178(6):1177-88)

– synergizes with TNF alpha against MmB16 melanoma (Neoplasma. 1995;42(2):69-74)

… and the list goes on.

My take

Notwithstanding occassional contradictory reports of statins increasing the risk of cancer, I feel strongly that given the safety (simvastatin is available as an OTC in the U.K.) and low cost of statins, plus the wide array of studies and accumulating data showing a protective effect of statins against cancer development and recurrence, statins should be seriously considered as part of a cocktailed approach for primary and secondary cancer prevention (especially for colon, breast, lung and prostate – where the data is strongest).  It should also be seriously considered as a cornerstone ingredient to combine synergistically with other compounds such as gamma tocotrienols, cox-2 inhibitors, bisphosphonates etc for added effects in cancer treatment.  Not all statins are the same however, and some (e.g. lipophilic statins such as simvastatin) may work better against certain cancers than others (e.g. hydrophilic statins such as pravastatin).  Dosage may be important as well.  Unfortunately, because most of the statins have patents that are expired or near expiration, there is a lack of incentive on the part of drug companies to conduct large scale clinical trials using these agents against cancer, so it is not clear that we will gain much more useful clinical insight in the near future, but I believe that these are nearly no-brainer drugs to add to most cancer preventative or treatment cocktails unless side-effects are an issue in an individual patient. Your comments welcome.

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Disulfiram (Antabuse) for cancer

November 24, 2008 1 comment

Disulfiram (Antabuse) is an old thiocarbamate drug known to support the treatment of alcoholism and cocaine dependency. Unlike Naltrexone which is also used for alcoholism, its potential usefulness is cancer is well researched but curiously little publicized (Naltrexone is conversely little researched and well publicized ! Will discuss in this blog).  I am in fact quite impressed by the numerous documented investigations into Disulfiram’s use as an anti-cancer when I first investigated its potential in this area.

Schirmer and Scott seemed to have been the earliest to notice a relationship of disulfiram and tumor inhibition (Trans Am Assoc Genitourin Surg. 1966;58:63-6.), and subsequently Wattenberg was able to demonstrate that the dietary disulfiram inhibited DMH chemical induction of bowel cancer in mice  (JNCI, 1975 Apr;54(4):1005-6). Eventual research since the 70s have demontrated that disulfiram can block the P-glycoprotein extrusion pump and thus reduce drug-resistance, inhibits the transcription factor nuclear factor-kappaB, reduces angiogenesis, and inhibits tumor growth in cell lines and rodents.

Now where is the evidence?

a) In Vitro (cellular evidence):

Disulfiram inactivates the ability of the Rous sarcoma virus to malignantly transform chick embryo cells.

Disulfiram potentiated the cytotoxicity of nitrogen mustard and 5FU chemotherapy effects on leukemia and colorectal cells respectively.

Disulfiram can potentially reduce P-glycoprotein (P-gp) mediated drug resistance by inhibiting P-gp activity (possibly via cysteine modification) and/or by blocking its maturation (JNCI 2000 Jun 7;92(11):898-902).

Disulfiram induces apoptosis in melanoma cells (Mol Cancer Ther 2002 Jan;1(3):197-204).

Disulfiram inhibited invasion and angiogenesis of both tumor and endothelial cells possibly via interactions with MMP-2 and MMP-9 and inhibiting their proteolytic activity through a zinc related mechanism (Mol Pharmacol 2003 Nov;64(5):1076-84).

Disulfiram, as a member of the metal-chelating group of dithiocarbamate compounds, is able to bind with tumor cellular copper, forming an active complex with proteasome-inhibitory, apoptosis-inducing and anti-cancer activities. (Int J Mol Med 2007 Dec;20(6):919-25)

Disulfiram inhibited expression of metalloproteinases MMP-2 and MMP-9 and suppressed the invasion of human osteosarcoma cells. (J. Biochem Mol Biol 2007 Nov 30;40(6):1069-76)

b) In Vivo (animal evidence)

Dietary disulfiram inhibits chemical induction of intestinal (colon), bladder and breast cancer in rodents.

Disulfiram reduced ifosfamide-induced nephrotoxicity in rodents.

Disulfiram potentiated the cytotoxicity of nitrogen mustard chemotherapy in rodents (Cancer Res. 1989 Dec 1;49(23):6658-61).

c) Clinical (human experiences)

Roemeling et al. reported that human beings given 2 g of oral disulfiram at a particular time of day and high doses of cisplatin had lesser kidney toxicity. Disulfiram administration also apparently does not interfere with the antineoplastic activity of cisplatin (Chronobiol Intl 1986;3(1):55-64). Based on such evidence, a phase I and II study of cisplatinum and disulfiram was carried out which however overturned the hypothesis that disulfiram afforded nephroprotection in platinum chemotherapy (Am J Clin Oncol. 1990 Apr;13(2): 119 -24).

More recently in 2004, a first clinical report using a combination of oral zinc gluconate and disulfiram at approved doses for alcoholism induced >50% reduction in hepatic metastases and produced clinical remission in a patient with stage IV metastatic ocular melanoma, who has continued on oral zinc gluconate and disulfiram therapy for 53 continuous months with negligible side effects.

There is currently ongoing clinical trials of disulfiram with copper gluconate against liver cancer in Utah (ClinicalTrials.gov Identifier: NCT00742911) and of disulfiram as adjuvant against lung cancer in Israel (ClinicalTrials.gov Identifier: NCT00312819).

My take

This drug long known as a treatment for alcoholism found recent revival of interest as an anticancer and this has recently been reviewed by ZE Sauna et al. of the National Cancer Institute (Mol Biosyst 2005 Jul; 1(2): 127-34.  Epub 2005 May 26.)  Although there are no significant completed clinical trials to mention, both in vitro and animal data are supportive of its use, especially in melanoma and in conjunction with certain chemotherapies (eg 5FU), in cases of potential chemo-resistance, and as an anti-angiogenic perhaps in conjunction with Zinc gluconate.

It is not a totally hassle- free drug to prescribe though:

The initial dose is 500 mg for 1 to 2 weeks, followed by a maintenance dose of 250 mg (range 125 mg–500 mg) per day. The total daily dosage should not exceed 500 mg.  It is known to be a drug with moderate side-effects.  Of course, side-effects could be provoked if taken with alcohol, hence its use to support detox for alcohol dependence.  But other significant side-effects include hepatitis (1 case in 30,000 treated/yr), and neurologic. There are rare reports of psychosis and confusional states and peripheral nse effects, tiredness, headache and sleepiness are the most common.  Due to its CNS activity, drug-drug interaction is also an issue:

Drugs that may interact with disulfiram include, but are not limited to:

  • Bupropion (Wellbutrin IR/SR/XL, Amfebutamone)
  • Amphetamines (Adderall, Dexedrine, etc.)
  • Methylphenidate (Ritalin, Concerta, Focalin, etc.)
  • Cocaine (Occasionally used in dental procedures, and a known substance of abuse.)

The metabolism of other drugs may be inhibited by disulfiram, increasing their potential for toxic effects. Drugs known to have adverse effects when used concurrently with disulfiram include amitriptyline, isoniazid, and metronidazole (all with acute changes in mental state), phenytoin, some benzodiazepines, morphine, pethidine, and barbiturates.