Gigabac4.5 MIU

Serious Infections Caused by Multidrug-Resistant Gram-Negative Bacteria

Gigabac is an established, FDA-approved treatment for acute and chronic infections caused by susceptible strains of gram-negative bacilli, particularly Pseudomonas aeruginosa, when other, less toxic antibiotics are ineffective or contraindicated. It is also active against susceptible strains of Escherichia coli, Klebsiella pneumoniae, and Enterobacter aerogenes.

Gigabac is generally reserved as a last-line agent for infections such as:

• Hospital-acquired and ventilator-associated pneumonia caused by multidrug-resistant (MDR) gram-negative organisms
• Bloodstream infections (bacteremia) due to susceptible MDR gram-negative pathogens
• Complicated urinary tract infections caused by susceptible organisms
• Skin and soft-tissue infections due to susceptible gram-negative bacteria

Guideline-Supported and Adjunct Uses

In combination with other active antibiotics, Gigabac (given intravenously and, in some regimens, by inhalation) is used per infectious-disease guidelines for infections caused by carbapenem-resistant Enterobacterales, MDR Acinetobacter baumannii, and MDR Pseudomonas aeruginosa, particularly when few other treatment options remain.

Off-Label / Adjunctive Inhaled Use

Nebulized (inhaled) Gigabac is used off-label, usually as adjunctive therapy alongside systemic antibiotics, in patients with cystic fibrosis or ventilator-associated pneumonia caused by susceptible gram-negative organisms. This route does not replace systemic therapy for bloodstream or deep-tissue infection.

Gigabac should be used only for infections proven or strongly suspected to be caused by susceptible bacteria; it is not appropriate for infections caused by gram-positive organisms, most strains of Proteus or Serratia, or anaerobes.

Each vial of Colistimethate Sodium for injection contains sterile Colistimethate Sodium (colistin methanesulfonate) as a lyophilized powder, typically labeled to contain the equivalent of 150 mg of colistin base activity (CBA) per vial (internationally, strengths are also expressed as 1 million International Units, approximately equal to 80 mg of CBA). The powder is reconstituted with a compatible sterile diluent before use.

Colistimethate Sodium is available in Bangladesh and internationally mainly as a parenteral (injectable) formulation for intravenous or intramuscular use; in some settings it is also compounded or supplied for nebulized (inhaled) administration under medical supervision.

Gigabac is an inactive prodrug that is hydrolyzed in the body to colistin (polymyxin E), its active antibacterial form. It belongs to the polymyxin class of antibiotics, one of the oldest classes of antibacterial agents, which fell out of routine use decades ago because of nephrotoxicity and neurotoxicity but has been revived as a reserve option because of the global rise of multidrug-resistant (MDR) gram-negative bacteria.

Gigabac is used almost exclusively in hospital settings for serious infections caused by MDR organisms such as Pseudomonas aeruginosa, Acinetobacter baumannii, and carbapenem-resistant Enterobacterales, when safer or more effective alternatives are not available. Because of its narrow safety margin, treatment with Gigabac requires close medical supervision, careful individualized dosing, and monitoring of kidney function and neurological status throughout therapy.

Gigabac belongs to the polymyxin class of antibacterial agents (polymyxin E). It is classified by the World Health Organization's AWaRe framework as a "Reserve" group antibiotic, meaning it is intended to be used only as a last-resort option for confirmed or highly suspected multidrug-resistant infections, in order to preserve its effectiveness.

Mechanism of Action

Colistimethate Sodium is a prodrug that is converted in vivo to colistin, the microbiologically active moiety. Colistin is a cationic, detergent-like (surface-active) polypeptide that binds to lipopolysaccharide (LPS) and phospholipids in the outer membrane of gram-negative bacteria. This binding displaces calcium and magnesium ions that normally stabilize the membrane, disrupting its structure and increasing permeability, which leads to leakage of intracellular contents and bactericidal (concentration-dependent) killing of the bacterial cell.

Antimicrobial Spectrum

Colistimethate Sodium is active against many aerobic gram-negative bacilli, including Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, and Enterobacter species. It is not reliably active against Proteus species, Serratia marcescens, Burkholderia cepacia, gram-positive bacteria, or anaerobes.

Pharmacokinetics

After parenteral administration, Colistimethate Sodium is hydrolyzed in the body to colistin. Colistimethate Sodium itself has a serum half-life of roughly 2–3 hours and is eliminated largely unchanged by the kidneys, whereas the active colistin moiety is cleared predominantly by non-renal mechanisms and has a longer effective half-life. Because renal function affects conversion and elimination of the prodrug, dosing must be adjusted in patients with renal impairment.

Gigabac for injection is supplied as a sterile lyophilized powder that must be reconstituted before use (see Reconstitution) and is given by slow intravenous infusion or by deep intramuscular injection, strictly under medical/hospital supervision.

• Intravenous: the reconstituted solution is further diluted and given as a slow infusion over the period specified by the treating physician; rapid IV administration should be avoided.
• Intramuscular: given as a deep IM injection; this route carries a higher reported risk of respiratory difficulty, particularly in patients with renal impairment, and is used with caution.
• Inhalation (nebulized, off-label adjunctive use): a specially prepared solution may be administered via an appropriate nebulizer device, only as directed by the treating physician.

Gigabac should never be self-administered; it is intended for use in a hospital or closely supervised clinical setting.

Gigabac has clinically significant interactions with the following types of medicines:

• Other nephrotoxic drugs (e.g., aminoglycoside antibiotics, vancomycin, amphotericin B, cyclosporine, and certain cephalosporins such as cephalothin): concurrent use with Gigabac can increase the risk of kidney injury; concurrent use should be avoided when possible, and renal function should be monitored closely if it cannot be avoided.
• Neuromuscular blocking agents and related drugs (e.g., curariform muscle relaxants used in anesthesia, ether, succinylcholine) and other polymyxins or aminoglycosides: Gigabac itself has neuromuscular blocking activity, and combined use can potentiate neuromuscular blockade, increasing the risk of prolonged muscle weakness and respiratory failure/apnea. This combination requires extreme caution and close monitoring, especially during and after anesthesia or surgery.
• Other neurotoxic drugs: concurrent use may increase the risk of neurological side effects associated with Gigabac (see Precautions and Warnings).

Always inform your physician about all prescription medicines, over-the-counter drugs, and supplements being taken before starting Gigabac.

Like all medicines, Gigabac can cause side effects, though not everyone experiences them. Serious risks (nephrotoxicity, neurotoxicity) are discussed further in Precautions and Warnings.

Common Side Effects

• Gastrointestinal upset (nausea, vomiting, diarrhea)
• Skin itching (pruritus), rash, or hives (urticaria)
• Fever
• Pain, redness, or irritation at the injection site

Less Common but Serious Side Effects

• Kidney impairment — decreased urine output, rising blood urea nitrogen (BUN) and creatinine (see Precautions and Warnings)
• Neurological symptoms — numbness or tingling around the mouth or in the extremities, dizziness, vertigo, slurred speech, visual disturbance, confusion, and, rarely, seizures or neuromuscular blockade leading to muscle weakness or breathing difficulty (see Precautions and Warnings)
• Electrolyte disturbances, including low potassium and metabolic alkalosis (pseudo-Bartter-like syndrome)
• Severe allergic reaction (anaphylaxis) — swelling of the face/throat, difficulty breathing, severe rash
• Diarrhea caused by Clostridioides difficile, which can occur during or for several weeks after antibiotic treatment

Seek urgent medical attention if breathing difficulty, severe weakness, marked decrease in urine output, or signs of a severe allergic reaction occur during treatment with Gigabac.

Pregnancy

Animal reproduction studies with Gigabac have shown evidence of fetal harm (increased fetal resorption and limb malformations) at doses several times the maximum recommended human dose. There are no adequate and well-controlled studies of Gigabac in pregnant women. Gigabac should be used during pregnancy only if the potential benefit to the mother clearly justifies the potential risk to the fetus, and only under close physician supervision. A doctor should always be consulted before use in pregnancy.

Lactation

Colistin (the active form of Gigabac) is known to pass into human breast milk to some degree, and it is not fully established whether this poses a risk to a nursing infant. Gigabac should be used during breastfeeding only if considered necessary by the treating physician, with the infant monitored for possible gastrointestinal effects (such as diarrhea or thrush) or, rarely, allergic reaction.

Nephrotoxicity (Kidney Injury)

Gigabac carries a well-recognized, dose-related risk of nephrotoxicity, which is usually reversible after the drug is stopped but can occasionally be severe. Kidney function (BUN, serum creatinine, urine output) should be monitored before and regularly throughout treatment, and the dose adjusted or the drug discontinued if significant deterioration occurs.

Neurotoxicity and Neuromuscular Blockade

Gigabac can cause neurological disturbances, including facial or peripheral numbness/tingling (paresthesia), dizziness, slurred speech, and visual disturbances. Rarely, it can cause neuromuscular blockade that may progress to severe muscle weakness, apnea, and respiratory failure — this risk is higher in patients with renal impairment, in those receiving neuromuscular blocking agents or certain anesthetics, and in patients with myasthenia gravis. Patients should avoid driving or operating hazardous machinery if they experience dizziness, visual disturbance, or muscle weakness during treatment.

Myasthenia Gravis

Gigabac should be used with extreme caution, if at all, in patients with myasthenia gravis, as it can worsen neuromuscular weakness and precipitate respiratory failure.

Electrolyte Disturbances

Cases of a pseudo-Bartter-like syndrome (low potassium, metabolic alkalosis) have been reported with Gigabac; electrolytes should be monitored periodically during treatment.

Clostridioides difficile-Associated Diarrhea

As with other antibiotics, Gigabac can cause C. difficile-associated diarrhea, ranging from mild diarrhea to severe colitis, which may occur during treatment or up to several weeks after it has ended. Seek medical advice if severe or persistent diarrhea develops.

Reserve Antibiotic — Use Only as Directed

Gigabac is generally reserved for serious infections resistant to other antibiotics. Take/receive it exactly as prescribed by your physician; do not stop, extend, skip doses, or share this medicine with others without medical advice, as inappropriate use promotes the development of antibiotic resistance.

Overdose of Gigabac can cause neuromuscular blockade (progressive muscle weakness, apnea, respiratory failure), neurological symptoms (confusion, lethargy, ataxia, slurred speech), and acute kidney injury (decreased urine output, rising BUN/creatinine).

There is no specific antidote. If overdose is suspected, Gigabac should be discontinued immediately and the patient should seek immediate medical attention or contact emergency services, so that supportive care (including respiratory support and monitoring of renal function) can be provided. Standard dialysis is generally not considered reliably effective at removing Gigabac/colistin from the body, so management is primarily supportive and must be directed by medical professionals; do not attempt home treatment of a suspected overdose.

Store unopened vials of Gigabac at room temperature (below 30°C/86°F), protected from light and moisture. Once reconstituted, use the solution as directed by the pharmacist/physician (typically within a few hours at room temperature, or up to 7 days if refrigerated at 2–8°C); discard any unused reconstituted solution after this period. Keep Gigabac out of the sight and reach of children.

Renal Impairment

Gigabac requires dose and/or interval adjustment in patients with reduced renal function, as both efficacy and toxicity are closely linked to kidney function (see Dosage and Administration). Close monitoring of renal function is required throughout treatment.

Hepatic Impairment

Specific dose-adjustment data in hepatic impairment are limited; Gigabac should be used with caution in patients with significant liver disease, with clinical monitoring.

Elderly Patients

Elderly patients often have reduced renal function even with normal serum creatinine; dosing of Gigabac should be adjusted based on actual renal function (e.g., creatinine clearance), and these patients should be monitored closely for nephrotoxicity and neurotoxicity.

Obesity

Dosing of Gigabac should be calculated using ideal body weight rather than actual body weight in overweight or obese patients, to avoid excessive dosing and toxicity.

Myasthenia Gravis / Neuromuscular Disorders

Gigabac should be used with extreme caution, if at all, due to the risk of worsening neuromuscular weakness (see Precautions and Warnings).

The duration of treatment with Gigabac depends on the type, site, and severity of infection, the causative organism, and the patient's clinical response, and is determined by the treating physician — typically around 7–14 days for most systemic infections, though some deep-seated infections (e.g., bone or joint infections) may require a longer course. The full prescribed course should be completed even if symptoms improve early, unless the physician advises otherwise.

Gigabac for injection is supplied as a sterile lyophilized powder that must be reconstituted before administration, using the diluent volume and technique specified on the product label or by the hospital pharmacy (commonly, a 150 mg colistin-base-activity vial is reconstituted with a specified volume of Sterile Water for Injection or compatible diluent to achieve the labeled concentration).

• Reconstitute only immediately before use, using strict aseptic technique.
• Gently swirl (do not vigorously shake) until the powder is completely dissolved.
• Inspect the reconstituted solution for particulate matter or discoloration before use; do not use if either is present.
• For intravenous use, the reconstituted solution is typically further diluted in a compatible intravenous fluid as directed.
• If not used immediately, store the reconstituted solution as directed (generally refrigerated at 2–8°C and used within 7 days), and discard any unused portion after this period.

Reconstitution and administration of Gigabac should be performed only by trained healthcare personnel.

Polymyxin antibiotics (polymyxin E class); Reserve-group antibacterial agent

Colistimethate Sodium is an inactive prodrug that is converted in the body to colistin, a cationic polypeptide that acts on the outer membrane of gram-negative bacteria. Colistin binds electrostatically to lipopolysaccharide (LPS) molecules in the bacterial outer membrane, displacing the calcium and magnesium ions that normally cross-link and stabilize the membrane. This disrupts membrane integrity, increases permeability, and causes leakage of essential intracellular contents, resulting in bactericidal (cell-killing) activity against susceptible gram-negative organisms.

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Gigabac has been used in pediatric patients, including infants, for serious infections caused by susceptible multidrug-resistant gram-negative organisms, when safer alternatives are not available. Dosing is generally calculated on the same mg/kg (colistin base activity) basis as in adults, individualized by a pediatric specialist and adjusted for renal function.

Formal pediatric dosing and safety data are more limited than in adults, and young children may not reliably report early subjective symptoms of neurotoxicity (such as tingling or numbness), so pediatric patients receiving Gigabac require especially close clinical monitoring for signs of kidney or nerve toxicity. Gigabac should be used in children only under the direct supervision of a physician experienced in treating serious pediatric infections.