The edition · Clinical Pharmacology
One person in 330 carries a variant that turns an aminoglycoside into permanent deafness
A UK pharmacogenomics guideline sets out when to test MT-RNR1 before gentamicin or amikacin — and is explicit that a missing result must not delay treatment in sepsis. Rifampicin removes 99% of a CYP3A substrate's exposure, and Baxter's premixed GALAXY containers are under a Class II recall spanning amiodarone and vancomycin.
The edition in brief
Clinical pharmacology today is dominated by two prescribing guidelines from the UK Centre of Excellence in Regulatory Science and Innovation in Pharmacogenomics. The first concerns MT-RNR1, a mitochondrial variant carried by roughly 1 in 330 people, which predisposes to irreversible sensorineural hearing loss after aminoglycoside exposure — sometimes after a single dose, and at therapeutic concentrations that therapeutic drug monitoring would call safe. The guideline recommends avoidance at any detectable variant level, notes that about 20% of aminoglycoside use is predictable enough for pre-emptive testing, and states plainly that where a result is unavailable and clinical urgency is high, treatment should not be delayed. The second guideline covers UGT1A1 testing before irinotecan, recommending a 30% dose reduction at cycle 1 in poor metabolisers with titration thereafter against tolerability and neutrophil counts. A three-arm interaction study in healthy subjects puts hard numbers on CYP3A4 modulation of the phosphodiesterase type 5 inhibitor aildenafil. Clarithromycin raised exposure 2.58-fold; rifampicin reduced it to one-hundredth of control, effectively abolishing the drug. Cimetidine did nothing. The rifampicin result matters disproportionately in Indian practice, where rifampicin exposure through tuberculosis treatment is common and its reach across CYP3A substrates is easy to forget. On the regulatory side, the US Food and Drug Administration lists three ongoing Class II recalls of Baxter premixed GALAXY single-dose containers for current good manufacturing practice deviations, covering Nexterone (amiodarone) and two vancomycin presentations. Class II means temporary or medically reversible harm is possible; this is a manufacturing action, not a signal about the drugs themselves.
Avoid aminoglycosides at any detectable MT-RNR1 variant — but never delay sepsis treatment for the result
Treat a maternal family history of hearing loss as a reason to choose a non-aminoglycoside where one will do — but never hold the antibiotic in sepsis waiting for a genotype.
Class II recall on Baxter premixed GALAXY containers, covering amiodarone and vancomycin
Treat this as a supply and preparation problem, not a drug safety problem — the risk arrives when the premixed bag is replaced by a locally compounded one.
Rifampicin removed 99% of a CYP3A substrate's exposure; clarithromycin raised it 2.6-fold
Before you escalate the dose of a CYP3A substrate that is not working, check whether the patient is on rifampicin.
Pre-emptive genotyping only helps where the prescription can be foreseen
Before commissioning a pharmacogenomic test, ask whether the prescribing decision it informs is ever made with time to wait for it.
Reduce irinotecan by 30% at cycle 1 in UGT1A1 poor metabolisers
Genotype before the first irinotecan dose and cut cycle 1 by 30% in poor metabolisers, then titrate up on counts.
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