Indications: The polymyxin antibiotic, colistimethate

sodium, is active against Gram-negative organisms including Pseudomonas aeruginosa, Acinetobacter baumanii, and Klebsiella pneumoniae. It is not absorbed by mouth and thus needs to be given by injection for a systemic effect. Intravenous administration of colistimethate sodium should be reserved for Gram- negative infections resistant to other antibacterial; its major adverse effects are dose-related neurotoxicity and nephrotoxicity.

Colistimethate sodium is also given by inhalation as an adjunct to standard antibacterial therapy in patients with cystic fibrosis.

Cautions: acute porphyria Specific cautions for parenteral treatment Monitor renal function Specific cautions for inhaled treatment Other inhaled drugs should be administered before colistimethate sodium. Measure lung function before and after initial dose of colistimethate sodium and monitor for bronchospasm; if bronchospasm occurs in a patient not using a bronchodilator, repeat test using a bronchodilator before the dose of colistimethate sodium. Severe haemoptysis (risk of further haemorrhage).

Contra-indications: myasthenia gravis

Renal impairment reduce dose and monitor plasma- colistimethate sodium concentration during parenteral treatment.

Pregnancy clinical use suggests probably safe when used by inhalation (use parenteral treatment only if potential benefit outweighs risk).

Breast-feeding present in milk but poorly absorbed from gut; manufacturers advise avoid (or use only if potential benefit outweighs risk).

Side Effects: Sore throat, sore mouth, taste disturbances, nausea, vomiting, cough, bronchospasm, dysphonia; less commonly thirst, hypersalivation.

Dose: By slow intravenous injection into a totally implantable venous access device, or by intravenous infusion (but see notes above), adult and child body- weight under 60 kg, 50 000–75 000 units/kg daily in 3 divided doses; body-weight over 60 kg, 1–2 million units every 8 hours.

NOTE: Plasma concentration monitoring recommended in renal impairment; recommended ‘peak’ plasma- colistimethate sodium concentration (approx. 1 hour after intravenous injection or infusion) 5–15 mg/litre, pre-dose (‘trough’) concentration 2–6 mg/litre.

By inhalation of nebulised solution, adult and child over 2 years, 1–2 million units twice daily; increased to 2 million units 3 times daily for subsequent respiratory isolates of Ps. aeruginosa; child 1 month–2 years, 0.5–1 million units twice daily; increased to 1 million units 3 times daily for subsequent respiratory isolates of Ps. aeruginosa

By inhalation of powder, adult and child over 6 years, 1.66 million units twice daily.