R-P MYCIN
Clinical safety rating: caution
Comprehensive clinical and safety monograph for R-P MYCIN (R-P MYCIN).
R-P MYCIN is a macrolide antibiotic that inhibits bacterial protein synthesis by binding to the 50S subunit of the bacterial ribosome, specifically at the 23S rRNA of the peptidyl transferase center. This action blocks the translocation step, thereby preventing the elongation of the peptide chain.
| Metabolism | R-P MYCIN is primarily metabolized by the hepatic cytochrome P450 system, mainly via CYP3A4 isoenzyme. It undergoes demethylation and other oxidative processes. Significant first-pass metabolism occurs, leading to variable bioavailability. |
| Excretion | Renal (60-80% unchanged), biliary/fecal (15-20%). |
| Half-life | Terminal half-life 2-3 hours; prolonged in renal impairment (up to 6-8 hours in anuria). |
| Protein binding | 30-40% bound to serum albumin. |
| Volume of Distribution | 0.6-1.2 L/kg; increased in extracellular fluid expansion or decreased in dehydration. |
| Bioavailability | Oral: 30-40% (due to first-pass metabolism); IV: 100%. |
| Onset of Action | Oral: 1-2 hours; Intravenous: immediate (within minutes). |
| Duration of Action | 6-8 hours (oral); 8-12 hours (IV) depending on dose and infection site. |
| Molecular Weight | 323.13 |
Rifampin 600 mg orally once daily or 10 mg/kg intravenously once daily.
| Dosage form | TABLET, DELAYED RELEASE |
| Renal impairment | No dose adjustment required for GFR >30 mL/min. For GFR 10-30 mL/min, reduce dose to 300 mg orally once daily. For GFR <10 mL/min, administer 300 mg orally once daily with monitoring. |
| Liver impairment | Child-Pugh Class A: standard dosing. Child-Pugh Class B: reduce to 450 mg orally once daily. Child-Pugh Class C: reduce to 300 mg orally once daily or avoid use. |
| Pediatric use | Children: 10-20 mg/kg orally once daily, maximum 600 mg/day. For tuberculosis, 10-20 mg/kg orally once daily; for prophylaxis, 15-20 mg/kg orally once daily. |
| Geriatric use | No specific dose adjustment required, but monitor for hepatotoxicity and adjust based on renal function. Consider starting at lower end of dosing range due to age-related decline in organ function. |
| 1st trimester | Avoid; risk of fetal harm based on animal data and limited human studies. May cause congenital malformations like cleft palate. |
| 2nd trimester | Avoid; risk of fetal harm, including potential for fetal hepatotoxicity and acute liver failure. |
| 3rd trimester | Avoid; risk of fetal harm, including gray baby syndrome (cyanosis, cardiovascular collapse, death) due to immature liver function. |
Clinical note
Comprehensive clinical and safety monograph for R-P MYCIN (R-P MYCIN).
| Placental transfer | Crosses placenta readily; fetal serum concentrations may reach 30-80% of maternal levels. |
| Breastfeeding | Excreted into breast milk; potential for serious adverse reactions in nursing infants, including bone marrow suppression and gray baby syndrome. Breastfeeding is contraindicated during therapy and for 48 hours after last dose. |
■ FDA Black Box Warning
There is no FDA black box warning for R-P MYCIN.
| Serious Effects |
Hypersensitivity to chloramphenicol or any componentPregnancyBreastfeedingSevere hepatic impairmentHistory of bone marrow suppression or blood dyscrasiasConcurrent use of drugs that may cause myelosuppression
| Precautions | Potential for QT interval prolongation and risk of cardiac arrhythmias, especially in patients with pre-existing cardiac conditions or electrolyte imbalances, Risk of hepatotoxicity, including cholestatic hepatitis and hepatic necrosis, Possible exacerbation of myasthenia gravis, Increased risk of infantile hypertrophic pyloric stenosis in neonates exposed in utero or during breastfeeding, Drug interactions due to CYP3A4 inhibition, leading to increased levels of statins, warfarin, and other drugs, May cause false elevations in urinary catecholamines and certain liver enzymes |
| Food/Dietary | Avoid alcohol. No specific food restrictions, but high-fat meals may delay absorption of rifampin. Maintain adequate hydration to reduce uric acid crystallization risk from pyrazinamide. |
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| Lactation Rating | L5 |
| Teratogenic Risk | FDA Pregnancy Category D. First trimester: Avoid due to risk of fetal harm (e.g., skeletal dysplasia, cardiovascular malformations). Second/third trimester: Use only if benefit outweighs risk; may cause fetal renal impairment, oligohydramnios, and neonatal toxicity. |
| Fetal Monitoring | Baseline renal function (serum creatinine, BUN), audiometry, and hepatic panel. During therapy: weekly renal function, urine output, fetal ultrasound for oligohydramnios, and therapeutic drug monitoring of trough levels (target <2 µg/mL). |
| Fertility Effects | May impair fertility in males and females based on animal studies (testicular atrophy, reduced ovarian function). Human data limited; potential for reversible infertility. |
| Clinical Pearls | R-P MYCIN is a combination of rifampin and pyrazinamide, used for tuberculosis. Monitor for hepatotoxicity; baseline and periodic LFTs required. Rifampin induces CYP450, reducing efficacy of oral contraceptives, warfarin, and antiretrovirals. Pyrazinamide may cause hyperuricemia and gout flares. Administer on empty stomach for better absorption. |
| Patient Advice | Take this medication exactly as prescribed, usually daily for the first 2 months of TB treatment. · Take on an empty stomach, 1 hour before or 2 hours after meals. · May discolor urine, sweat, sputum, or tears orange-red (harmless). · Avoid alcohol completely due to increased risk of liver damage. · Use non-hormonal contraception; rifampin reduces birth control pill effectiveness. · Report signs of liver injury: yellowing skin/eyes, dark urine, abdominal pain, nausea/vomiting, unusual tiredness. · Report joint pain or swelling (pyrazinamide may raise uric acid). |