Reclast (Zoledronic Acid) and Metformin Interaction: What Patients and Clinicians Need to Know

Do Reclast and metformin interact directly?

The Reclast label states that no in vivo drug interaction studies have been performed. It also states that zoledronic acid is not metabolized and is excreted into urine as the intact drug, and that in vitro studies show it does not inhibit human CYP450 enzymes [1]. Metformin is also not metabolized; it is negligibly bound to plasma proteins and is cleared by the kidneys [2].

What both labels do describe is a shared dependence on the kidney, approached from opposite directions.

Where the two drugs meet: renal clearance

Zoledronic acid is eliminated unchanged by the kidney. In animals less than 3% appeared in feces, with the rest excreted in urine or taken up by bone, and the terminal elimination half-life is 146 hours [1]. Metformin is excreted unchanged in urine by tubular secretion, with a renal clearance about 3.5 times creatinine clearance; its half-life is prolonged and its clearance falls as renal function declines [2].

The Reclast label addresses renally excreted drugs directly. Its interactions section says that in patients with renal impairment, exposure to concomitant medications that are primarily renally excreted (it gives digoxin as the example) may increase, and that monitoring serum creatinine should be considered in at-risk patients taking such drugs [1]. Metformin is primarily renally excreted [2].

The metformin label approaches the same ground from the other side. Its lactic acidosis warnings say that concomitant use of drugs that impair renal function, cause significant hemodynamic change, interfere with acid-base balance, or increase metformin accumulation may raise the risk, and that more frequent monitoring of those patients should be considered [2]. The Reclast label, meanwhile, says the drug should be used with caution alongside other potentially nephrotoxic drugs such as NSAIDs, and warns that acute renal failure has followed a single dose [1].

What the Reclast label says around each dose

The Reclast label says:

• Creatinine clearance should be calculated using the Cockcroft-Gault formula and actual body weight before each dose [1].
• Reclast is contraindicated at a creatinine clearance below 35 mL/min and in patients with evidence of acute renal impairment [1].
• Patients must be appropriately hydrated beforehand. If history or physical signs suggest dehydration, therapy should be withheld until normovolemic status is achieved [1].
• Elderly patients and those on diuretic therapy are at increased risk of acute renal failure and should have fluid status assessed and be hydrated before administration [1].
• The dose is 5 mg infused over no less than 15 minutes, followed by a 10 mL normal saline flush [1].

What the Reclast trials measured

Study 1 enrolled 7,736 postmenopausal women aged 65 to 89. Of these, 3,862 received Reclast and 3,852 received placebo; treatment was given annually for three years [1]. A transient increase in serum creatinine within 10 days of dosing occurred in 1.8% of Reclast-treated patients versus 0.8% of placebo-treated patients, and resolved without specific therapy [1]. Across the 3 years, the change in creatinine clearance and the incidence of renal failure and impairment were comparable between the two groups, including in patients whose baseline creatinine clearance was 30 to 60 mL/min [1].

The warning language covers the less common course. Acute renal failure has been observed after a single administration, with rare reports of hospitalization, dialysis or fatal outcome, in patients with underlying moderate to severe renal impairment or with risk factors such as advanced age, dehydration, diuretic therapy or nephrotoxic drugs [1].

What the metformin label says about the kidney and lactate

Metformin hydrochloride carries a boxed warning for lactic acidosis. Postmarketing cases have resulted in death, hypothermia, hypotension and resistant bradyarrhythmias, and the onset is often subtle, with malaise, myalgias, respiratory distress, somnolence and abdominal pain [2]. The laboratory picture is blood lactate above 5 mmol/L, anion gap acidosis, and metformin plasma levels generally above 5 mcg/mL [2].

The label's renal rules are specific. Metformin is contraindicated at an eGFR below 30 mL/min/1.73 m2; starting it is not recommended at an eGFR of 30 to 45; if eGFR later falls below 45 the benefit and risk of continuing should be assessed; and it should be discontinued if eGFR falls below 30 [2]. The label also states that metformin decreases liver uptake of lactate, which raises blood lactate levels and may increase lactic acidosis risk, especially in patients at risk [2].

Does the metformin label cover a bisphosphonate infusion?

The label's procedure-specific instruction is written for iodinated contrast, not for bisphosphonate infusions. It says to stop metformin at the time of, or before, an iodinated contrast imaging procedure in patients with eGFR 30 to 60 mL/min/1.73 m2, a history of hepatic impairment, alcoholism or heart failure, or when intra-arterial contrast is used, then re-evaluate eGFR 48 hours later and restart if renal function is stable [2]. Separately, the label says metformin should be temporarily discontinued while patients have restricted food and fluid intake during surgery or other procedures, because withholding food and fluids may increase the risk of volume depletion, hypotension and renal impairment [2].

Its interaction table names carbonic anhydrase inhibitors such as topiramate and zonisamide, drugs that reduce metformin clearance (ranolazine, vandetanib, dolutegravir, cimetidine), alcohol, insulin and insulin secretagogues, and drugs that raise blood glucose; zoledronic acid is not on that list [2].

Putting the two labels together

Reclast is contraindicated at a creatinine clearance below 35 mL/min or with acute renal impairment, while metformin is contraindicated at an eGFR below 30 mL/min/1.73 m2 [1][2]. The Reclast label calls for a calculated creatinine clearance before every dose and says to consider creatinine monitoring in at-risk patients taking renally excreted drugs [1]. The metformin label says to consider more frequent monitoring when another drug can impair renal function, and to check eGFR at least yearly in everyone taking metformin [2].