Anthracyclines and Intercalation Inhibitors: Doxorubicin and Epirubicin

Mechanism

Pharmacological Class and Group

Anthracycline group antitumor antibiotics, non-specific phase of the cell cycle.

Mechanism of Action

Anthracyclines are planar aromatic compounds that exert their antitumor activity through three interconnected molecular mechanisms:

Mechanism of Intercalation and Inhibition of Topoisomerase II

1. Intercalation in DNA: The flat portions of anthraquinone slide perpendicularly between adjacent base pairs of DNA, distorting the spatial conformation of the double helix and blocking the synthesis of macromolecules.

2. Stabilization of the Cleavable Topoisomerase II Complex: They specifically bind to the DNA-Topoisomerase II ternary complex, preventing the sealing of the strands after physiological enzymatic cutting. This consolidates insoluble DNA double-strand breaks, inducing apoptosis:

Anthracycline + DNA-Topoisomerase II Complex Ligation Blocking DNA Double Strand Breaks

3. Generation of Free Radicals by Redox Cycle: The quinone ring undergoes a single electron reduction mediated by intracellular reductases, forming a semiquinone free radical. This reacts rapidly with molecular oxygen to generate superoxide anion radicals (O₂^-), which, due to the Fenton reaction in the presence of intracellular iron, produce hydroxyl radicals (OH) that are highly destructive to membrane lipids and organelles.

Pharmacokinetics

High Resolution Pharmacokinetics

• Distribution: It has an extraordinarily high volume of distribution (20 - 30 L/kg) due to rapid tissue uptake and avid binding to cellular DNA. It does not cross the blood-brain barrier intact.
• Metabolism: It undergoes extensive hepatic clearance. The drug is metabolized by cytosolic aldocetoreductases to its main active metabolite, doxorubicinol, which maintains the tissue cardiotoxicity of the original molecule.
• Elimination: Predominant hepatobiliary clearance; Approximately 80% of the dose is excreted in bile and feces as unchanged drug or active metabolites. Renal excretion less than 10%, temporarily coloring the urine reddish.
• Half-life (t_1/2): The plasma elimination half-life is approximately 20 to 48 hours.

Indicators and dose

Clinical Indications and Frequent Regimens

• Conventional Doxorubicin: Breast cancer (AC Scheme), Hodgkin lymphomas (ABVD Scheme) and non-Hodgkin lymphomas (CHOP), bone and soft tissue sarcomas.
• Pegylated Liposomal Doxorubicin (Caelyx): Formulation encapsulated in liposomes coated with methoxypolyethylene glycol. Indicated in recurrent ovarian cancer, HIV-associated Kaposi sarcoma and refractory multiple myeloma. It offers less cardiotoxicity and less alopecia, but increases acral erythema.
• Epirrubicin (FEC Scheme): Adjuvant breast cancer, dosed at 75 - 100 mg/m² every 21 days.

Cumulative Doses of Anthracyclines and the Role of Dexrazoxane

The risk of clinical congestive heart failure increases exponentially above a threshold cumulative dose. The following maximum cumulative limits must be strictly adhered to throughout the patient's life:

• Doxorubicin: Maximum cumulative dose of 450 - 550 mg/m². The risk of cardiomyopathy jumps from 5% to 26% if more than 550 mg/m² is exceeded.
• Epirrubicin: Maximum cumulative dose of 900 mg/m² (due to its different clearance kinetics and alternative glucuronide clearance).

In patients with an urgent need to continue treatment or with pre-existing cardiovascular risk factors, the cardioprotectant Dexrazoxane (an intracellular soluble iron chelating agent that interferes with the formation of the iron-anthracycline complex and transiently blocks Topoisomerase IIβ in cardiomyocytes) can be co-administered on a mandatory basis, reducing the incidence of chronic cardiac injury without reducing antitumor efficacy.

Dosage and Clinical Adjustment

• Standard IV doxorubicin: 60 - 75 mg/m² as a slow bolus or short infusion every 21 days.
• Adjustments in Liver Failure: Anthracyclines require strict adjustment based on total serum bilirubin due to their biliary clearance:
  • Bilirubin 1.2 - 3.0 mg/dL: Reduce the dose by 50%.
  • Bilirubin 3.1 - 5.0 mg/dL: Reduce the dose to 75%.
  • Bilirubin > 5.0 mg/dL: Administration is absolutely contraindicated.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Previous severe cardiac dysfunction (Left Ventricular Ejection Fraction [LVEF] < 50%).
• Total cumulative dose already achieved for the molecule.
• Severe liver failure (total bilirubin > 5 mg/dL).
• Severe uncontrolled myelosuppression.

Relative Contraindications

• Established coronary heart disease or severe arrhythmias under treatment.
• Previous concurrent thoracic or mediastinal radiotherapy.

Adverse Effects (ADR) and Specific Toxicity

• Hematological: Deep short nadir neutropenia (day 10-14).
• Cardiotoxicity: It is classified as acute (transient arrhythmias with a benign course in the first hours of infusion) and chronic cumulative. Chronic disease is associated with the development of congestive refractory dilated cardiomyopathy, dependent on the generation of free radicals and cardiomyocyte apoptosis due to damage to the Topoisomerase IIβ isoenzyme.
• Gastrointestinal: Rapid onset diffuse total alopecia, nausea, severe oropharyngeal mucositis.
• Tissue: Powerful vesicant agent. Accidental extravasation in peripheral soft tissues causes progressive local cellular necrosis, deep ulceration that is difficult to heal, and permanent tendon or neural injury.