cardiac drug interactions with statins

The American Heart Association just published their recommendations for managing drug-drug interactions between statins and other drugs used for cardiovascular disease (see statin drug interactions circ2016 in dropbox, or DOI: 10.1161/CIR.0000000000000456 ). I bring this up because statins are such an important drug that we use all the time, drug-drug interactions overall are common (leading to about 3% of hospital admissions, and the result of many annoying flags in electronic medical records) and because this review goes into detail on the mechanisms of these interactions (whether through the cytochrome P-450, CYP450, system of oxidative enzymes, or those interactions involving permeability glycoprotein P-gp, a superfamily of membrane-associated ATP-binding cassette-transporters, also called multidrug resistance-1; this article gives a good review of these systems, the latter of which was pretty much unknown to me prior to seeing this article….), and the article comments on the likely clinical importance of these interactions. Of note, I did not include recommendations for fluvastatin or pitavastatin, since these seem to be used rarely. Here is a brief review of the important/relatively common drugs used in primary care:

--amlodipine: minor interaction with lovastatin and simvastatin (for the latter, 1.8-fold increase in simvastatin level, with recommendation to limit simvastatin dose to 20mg)
--colchicine: variable interaction with all of the statins, though combination "may be considered". Monitor muscle toxicity closely. Of note, colchicine itself can cause myopathy, so can be hard to differentiate from statin-induced. Colchicine does not seem to impair CYP450 system though does seem to compete for P-gp-mediated efflux. Not much clinical data on the interaction with statins, but likely that best tolerated (based on the effect on P-gp) would be rosuvastatin, lovastatin, or pravastatin. And probably best to use lower doses of colchicine: 0.6-1.2 mg loading dose and 0.3-0.6 mg/d maintenance
--digoxin: not dependant on the CYP450 system. atorvastatin increases digoxin level 1.2-fold. Combo is reasonable with any statin without dose adjustment. But best to monitor digoxin levels in those on atorvastatin, esp if on higher doses.
--diltiazem: minor interaction with atorvastatin, but moderate with lovastatin and simvastatin (about 4-fold increase in statin levels). Limit lovastatin dose to 20mg and simvastatin to 10mg
--fenofibrate: insignificant interaction with all (though no data on lovastatin)
--gemfibrozil: minor interaction with atorvastatin and rosuvastatin. Avoid combo with lovastatin, pravastatin, and simvastatin
--verapamil: moderate interaction with lovastatin and simvastatin. Limit dose of lovastatin to 20mg and simvastatin to 10mg
--warfarin: 30% increase in INR with simvastatin, variable effects with lovastatin, rosuvastatin. But monitor INR with initiation or change in dose of any statin [which i think is reasonable with almost any new drug...]

A brief comment on cardiac and other drugs used less frequently in primary care:
--amiodarone: metabolized by both CYP34A and CYP2C8, as well as substrates of P-gp efflux transporter. Not use simvastatin (and definitely not more than 20mg) or lovastatin (not more than 40mg). rest are okay
--dronedarone: similar to amiodarone, but recommended to use simvastatin and lovastatin at even lower doses (eg not more than 10mg)
--ranolazine: best to use rosuvatatin, atorvastatin, pravastatin. FDA recommendation to limit dose of simvastatin to 20mg
--ticagrelor: can use atorvastatin; if use simvastatin or lovastatin, then no more than 40mg/d; no reported interactions with other statins
--conivaptan (a vasopressin receptor antagonist): not use lovastatin/simvastatin; best to use atorvastatin, pravastatin, or rosuvastatin. Tolvaptan can be used with any statin
--immunosuppressants: avoid combos of lovastatin or simvastatin with cyclosporine, everolimus, tacrolimus; best to use rosuvastatin up to 5mg, atorvastatin up to 10mg, pravastatin up to 20mg
--sacubitril/valsartan: limited data, minimal metabolism by CYP450 system, some increase in atorvastatin levels but no reported adverse clinical effects
--ivabradine: limited data, but extensively metabolized by CYP34A system (so, I would avoid simvastatin/lovastatin)

Commentary:
--this review was helpful to me for a few drug-drug interactions in particular
    --gemfibrozil: my prior held belief (and the 2013 American Heart Association guidelines statement) was that we should not use the combo of gemfibrozil with any statin, because of significant risk of myositis with the combo  (also some increased risk of hepatotoxicity). This created a bit of a problem, since studies do suggest that fenofibrate seems to be much less effective in lowering triglyceride levels and does not seem to have the same cardiovascular benefits of gemfibrozil (eg, comparing the FIELD trial of fenofibrate and the VA-HIT or Helsinki trials with gemfibrozil; also a meta-analysis confirmed that fenofibrate is a poor cousin to gemfibrozil). These new guidelines still prefer using fenofibrate, since large studies with the statin/fenofibrate combo do not show an increased toxicity; and they suggest using atorvastatin or rosuvastatin at a lower dose (FDA suggests max rosuvastatin of 10mg/d) if use gemfibrozil. But at least the combo of gemfibrozil and atorvastatin or rosuvastatin is now acceptable.
    ​--it really is okay to use simvastatin with amlodipine, despite my electronic record and pharmacist yelling at me when I have tried to do so. Though, that being said, it does seem that atorvastatin is overall better tolerated in terms of drug-drug interactions (rosuvastatin and pravastatin are also quite good). The reason is that simvastatin and lovastatin are metabolized in the CYP3A4 system, the most common enzyme system in drug metabolism, and atorvastatin is metabolized by this system less so. Rosuvastatin is metabolized in the less commonly used CYP2C9 system. Pravastatin is not metabolized through the CYP450 system at all, but is by the OATP enzymes (organic anion-transporting polyprotein)
--So, overall this guideline does present a more detailed (and complex) characterization of the drug-drug interactions between statins and other cardiac meds, and is pretty helpful with some very specific guidance. Unfortunately, it does not deal with non-cardiac meds (and, it turns out, that we in primary care do sometimes prescribe non-cardiac meds…)