Savvy Psychopharmacology

Protein binding changes and drug interactions: What do we know?

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Mr. S, age 47, weighs 209 lb and has a history of seizure disorder, bipolar disorder not otherwise specified, hypertension, and type 2 diabetes mellitus. He presents to the emergency department after not taking his medications for 2 days while on vacation. He has increased energy, decreased sleep, and pressured speech, and insists on walking for up to 10 hours per day “in preparation for a marathon,” even though he has a 4-cm foot ulcer. His family reports that he had been compliant with his medications until the present incident.

Mr. S has no known drug allergies. His medications include oral divalproex sodium delayed release (valproic acid [VPA]), 1,000 mg twice a day, oral lisinopril, 20 mg every morning, and insulin glargine, 22 units subcutaneously every evening.

A complete blood count, basic metabolic panel, creatine kinase level, VPA level, and urine drug screen are ordered. Relevant results include a serum creatinine level of 1.4 mg/dL (normal range: 0.6 to 1.2 mg/dL), a glucose serum level of 188 mg/dL (normal range: 70 to 100 mg/dL), and a VPA level of 23 mcg/mL (therapeutic range: 50 to 125 mcg/mL). A liver function panel is within normal limits: albumin level of 3.9 g/dL, aspartate aminotransferase level of 18 IU/L, and alanine aminotransferase level of 14 IU/L. In light of Mr. S’s seizure history, neurology is consulted and the decision is made to continue treating him with VPA because he has been seizure-free for 4.5 years and this medication has also helped with his bipolar disorder.

Mr. S is admitted to the hospital and his home medications are resumed at the current doses. On hospital Day 3, Mr. S’s VPA level is 62 mcg/mL, his obsession with a marathon has remitted, and his sleep pattern has normalized. Infectious disease and podiatry services are consulted for his diabetic foot infection, which has ulcerated down to the bone. IV ertapenem, 1,000 mg/d, is initiated with plans for debridement the following week. Two days later, Mr. S has a witnessed seizure; his VPA level is 9 mcg/mL.

A common question asked of pharmacists is, “Will protein binding changes affect drug dosages?” In this article, I describe how protein binding changes may occur, and the complexity of the dynamic. Being highly bound to a protein typically does not mean all medications will interact, but some interactions can be important. This article does not cover medications that bind to hormones.

Why is protein binding important? When a medication is bound to plasma protein, it is not free to act. There can be a delay in therapeutic effect (because no drug is available to react), delayed elimination, or possibly displacement of another protein-bound medication. Additionally, medications tend not to cross the blood-brain barrier or be eliminated when bound. For example, if a drug is 99% bound (leaving 1% free) and displacement now leaves 2% of the drug free, this event has doubled the amount of free drug. As the unbound medication is eliminated, the drug that is bound to the protein can act as a reservoir. A dynamic relationship exists between bound drug, unbound drug, and rate of elimination.

Which proteins do drugs commonly bind to? The proteins often associated with binding include albumin, alpha-1-acid glycoprotein (AAG), and lipoproteins. Albumin comprises 60% of total plasma protein in the plasma. Lipoproteins include very high-density lipoprotein (VHDL), high-density lipoprotein (HDL), very low-density lipoprotein (VLDL), and low-density lipoprotein (LDL).1 Medications that bind to lipoproteins include cyclosporine, tacrolimus, and propofol.2

Continued to: What common disease states can cause hypoalbuminemia?

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