Mechanism of action
Generally, doxorubicin is thought to exert its antineoplastic activity through 2 primary mechanisms: intercalation into DNA and disrupt topoisomerase-mediated repairs and free radicals-mediated cellular damages. Doxorubicin can intercalate into DNA through the anthraquinone ring, which stabilizes the complex by forming hydrogen bonds with DNA bases. Intercalation of doxorubicin can introduce torsional stress into the polynucleotide structure, thus destabilizing nucleosome structures and leading to nucleosome eviction and replacement. Additionally, the doxorubicin-DNA complex can interfere with topoisomerase II enzyme activity by preventing relegation of topoisomerase-mediated DNA breaks, thus inhibiting replication and transcription and inducing apoptosis. Moreover, doxorubicin can be metabolized by microsomal NADPH-cytochrome P-450 reductase into a semiquinone radical, which can be reoxidized in the presence of oxygen to form oxygen radicals. Reactive oxygen species have been known to cause cellular damage through various mechanisms, including lipid peroxidation and membrane damage, DNA damage, oxidative stress, and apoptosis. Although free radicals generated from this pathway can be deactivated by catalase and superoxide dismutase, tumor and myocardial cells tend to lack these enzymes, thus explaining doxorubicin's effectiveness against cancer cells and tendency to cause cardiotoxicity.
Doxorubicin hydrochloride is an antineoplastic antibiotic with pharmacologic actions similar to those of daunorubicin. Although the drug has anti-infective properties, its cytotoxicity precludes its use as an anti-infective agent. The precise and/or principal mechanism(s) of the antineoplastic action of doxorubicin is not fully understood. It appears that the cytotoxic effect of the drug results from a complex system of multiple modes of action related to free radical formation secondary to metabolic activation of the doxorubicin by electron reduction, intercalation of the drug into DNA, induction of DNA breaks and chromosomal aberrations, and alterations in cell membranes induced by the drug. Evidence from in vitro studies in cells treated with doxorubicin suggests that apoptosis (programmed cell death) also may be involved in the drug's mechanism of action. These and other mechanisms (chelation of metal ions to produce drug-metal complexes) also may contribute to the cardiotoxic effects of the drug.
Doxorubicin undergoes enzymatic 1- and 2-electron reduction to the corresponding semiquinone and dihydroquinone. 7-Deoxyaglycones are formed enzymatically by 1-electron reduction, and the resulting semiquinone free radical reacts with oxygen to produce the hydroxyl radical in a cascade of reactions; this radical may lead to cell death by reacting with DNA, RNA, cell membranes, and proteins. The dihydroquinone that results from 2-electron reduction of doxorubicin also can be formed by the reaction of 2 semiquinones. In the presence of oxygen, dihydroquinone reacts to form hydrogen peroxide, and in its absence, loses its sugar and gives rise to the quinone methide, a monofunctional alkylating agent with low affinity for DNA. The contribution of dihydroquinone and the quinone methide to the cytotoxicity of doxorubicin is unclear. Experimental evidence indicates that doxorubicin forms a complex with DNA by intercalation between base pairs, causing inhibition of DNA synthesis and DNA-dependent RNA synthesis by the resulting template disordering and steric obstruction. Doxorubicin also inhibits protein synthesis. Doxorubicin is active throughout the cell cycle including the interphase.
Several anthracycline-induced effects may contribute to the development of cardiotoxicity. In animals, anthracyclines cause a selective inhibition of cardiac muscle gene expression for ?-actin, troponin, myosin light-chain 2, and the M isoform of creatine kinase, which may result in myofibrillar loss associated with anthracycline-induced cardiotoxicity. Other potential causes of anthracycline-induced cardiotoxicity include myocyte damage from calcium overload, altered myocardial adrenergic function, release of vasoactive amines, and proinflammatory cytokines. Limited data indicate that calcium-channel blocking agents (eg, prenylamine) or beta-adrenergic blocking agents may prevent calcium overload ... It has been suggested that the principal cause of anthracycline-induced cardiotoxicity is associated with free radical damage to DNA.
Anthracyclines intercalate DNA, chelate metal ions to produce drug-metal complexes, and generate oxygen free radicals via oxidation-reduction reactions. Anthracyclines contain a quinone structure that may undergo reduction via NADPH-dependent reactions to produce a semiquinone free radical that initiates a cascade of oxygen-free radical generation. It appears that the metabolite, doxorubicinol, may be the moiety responsible for cardiotoxic effects, and the heart may be particularly susceptible to free-radical injury because of relatively low antioxidant concentrations. ... Chelation of metal ions, particularly iron, by the drug results in a doxorubicin-metal complex that catalyzes the generation of reactive oxygen free radicals, and the complex is a powerful oxidant that can initiate lipid peroxidation in the absence of oxygen free radicals. This reaction is not blocked by free-radical scavengers, and probably is the principal mechanism of anthracycline-induced cardiotoxicity.
The effect of doxorubicin on reactive oxygen metb in rat heart was investigated. It produced oxygen radicals in heart homogenate, sarcoplasmic reticulum, mitochondria, and cytosol, the major sites of cardiac damage. Superoxide prodn in heart sarcosomes and the mitochondrial fraction was incr. Apparently, free radical formation by doxorubicin, which occurs in the same myocardial compartments that are subject to drug-induced tissue injury, may damage the heart by exceeding the oxygen radical detoxifying capacity of cardiac mitochondria and sarcoplasmic reticulum.
Pharmacodynamics
Doxorubicin is a cytotoxic, cell-cycle non-specific anthracycline antibiotic. It is generally thought to exert its antitumor effect by destabilizing DNA structures through intercalation, thus introducing DNA strand breakages and damages. Not only does it alter the transcriptomes of the cells, failure in repairing DNA structures can also initiate the apoptotic pathways. Additionally, doxorubicin intercalation can also interfere with vital enzyme activity, such as topoisomerase II, DNA polymerase, and RNA polymerase, leading to cell cycle arrests. Finally, doxorubicin can also generate cytotoxic reactive oxygen species to exert cellular damages.
Pharmacokinetics
Half-life
The terminal half-life of doxorubicin ranges from 20 hours to 48 hours. The distribution half-life of doxorubicin is approximately 5 minutes. For the liposomal formulation, the first-phase and second-phase half-lives were calculated to be 4.7 ± 1.1 and 52.3 ± 5.6 hours respectively for a 10 mg/m<sup>2</sup> of doxorubicin in patients with AIDS-Related Kaposi’s Sarcoma.
Plasma concentrations of nonencapsulated doxorubicin and its metabolites decline in a biphasic or triphasic manner. In the first phase of the triphasic model, nonencapsulated doxorubicin is rapidly metabolized, presumably by a first-pass effect through the liver. It appears that most of this metabolism is completed before the entire dose is administered. In the triphasic model, nonencapsulated doxorubicin and its metabolites are rapidly distributed into the extravascular compartment with a plasma half-life of approximately 0.2-0.6 hours for doxorubicin and 3.3 hours for its metabolites. This is followed by relatively prolonged plasma concentrations of doxorubicin and its metabolites, probably resulting from tissue binding. During the second phase, the plasma half-life of nonencapsulated doxorubicin is 16.7 hours and that of its metabolites is 31.7 hours. In the biphasic model, the initial distribution t1/2 has been reported to average about 5-10 minutes, and the terminal elimination t1/2 has been reported to average about 30 hours.
Plasma concentrations of liposomally encapsulated doxorubicin hydrochloride appear to decline in a biphasic manner. Following iv administration of a single 10- to 40-mg/sq m dose of doxorubicin hydrochloride as a liposomal injection in patients with AIDS-related Kaposi's sarcoma, the initial plasma half-life (t1/2 alpha) of doxorubicin averaged 3.76-5.2 hours while the terminal elimination half-life (t1/2 beta) averaged 39.1-55 hours.
The initial distribution half-life of approximately 5 minutes suggests rapid tissue uptake of doxorubicin, while its slow elimination from tissues is reflected by a terminal half-life of 20 to 48 hours.
Plasma T/2 of Adriamycin is about 17 hr in patient, whereas that of its metabolites is about 32 hr.
Absorption
Following a 10 mg/m2 administration of liposomal doxorubicin in patients with AIDS-related Kaposi's Sarcoma, the Cmax and AUC values were calculated to be 4.12 ± 0.215 μg/mL and 277 ± 32.9 μg/mL•h respectively.
Approximately 40% of the dose appears in the bile in 5 days, while only 5% to 12% of the drug and its metabolites appear in the urine during the same time period. In urine, <3% of the dose was recovered as doxorubicinol over 7 days.
The steady-state distribution volume of doxorubicin ranges from 809 L/m<sup>2</sup> to 1214 L/m<sup>2</sup>.
The plasma clearance of doxorubicin ranges from 324 mL/min/m2 to 809 mL/min/m<sup>2</sup> by metabolism and biliary excretion. Sexual differences in doxorubicin were also observed, with men having a higher clearance compared to women (1088 mL/min/m<sup>2</sup> versus 433 mL/min/m<sup>2</sup>). Following the administration of doses ranging from 10 mg/m2 to 75 mg/m<sup>2</sup> of doxorubicin hydrochloride, the plasma clearance was estimated to be 1540 mL/min/m<sup>2</sup> in children greater than 2 years of age and 813 mL/min/m<sup>2</sup> in infants younger than 2 years of age.
Nonencapsulated doxorubicin hydrochloride is not stable in gastric acid, and animal studies indicate that the drug undergoes little, if any, absorption from the GI tract. The drug is extremely irritating to tissues and, therefore, must be administered iv. Following iv infusion of a single 10- or 20-mg/sq m dose of liposomal doxorubicin hydrochloride in patients with AIDS-related Kaposi's sarcoma, average peak plasma doxorubicin (mostly bound to liposomes) concentrations are 4.33 or 10.1 ug/mL, respectively, following a 15-minute infusion and 4.12 or 8.34 ug/mL, respectively, following a 30-minute infusion. Following iv infusion over 15 minutes of a 40-mg/sq m dose of liposomal doxorubicin hydrochloride in adults with AIDS-related Kaposi's, peak plasma concentrations averaged 20.1 ug/mL.
Nonencapsulated (conventional) doxorubicin hydrochloride exhibits linear pharmacokinetics; PEG-stabilized liposomal doxorubicin hydrochloride also exhibits dose-proportional, linear pharmacokinetics over a dosage range of 10-20 mg/sq m. The pharmacokinetics of liposomally encapsulated doxorubicin at a dose of 50 mg/sq m have been reported to be nonlinear. At a dose of 50 mg/sq m, a longer elimination half-life and lower clearance compared to those observed with a 20 mg/sq m dose are expected, with greater-than-proportional increases in area under the plasma concentration-time curve. Encapsulation of doxorubicin hydrochloride in PEG-stabilized (Stealth) liposomes substantially alters the pharmacokinetics of the drug relative to conventional iv formulations (ie, nonencapsulated drug), with resultant decreased distribution into the peripheral compartment, increased distribution into Kaposi's lesions, and decreased plasma clearance.
Metabolism
Doxorubicin is capable of undergoing 3 metabolic routes: one-electron reduction, two-electron reduction, and deglycosidation. However, approximately half of the dose is eliminated from the body unchanged. The two-electron reduction is the major metabolic pathway of doxorubicin. In this pathway, doxorubicin is reduced to doxorubicinol, a secondary alcohol, by various enzymes, including Alcohol dehydrogenase [NADP(+)], Carbonyl reductase [NADPH] 1, Carbonyl reductase [NADPH] 3, and Aldo-keto reductase family 1 member C3. The one-electron reduction is facilitated by several oxidoreductase, both cytosolic and mitochondrial, to form a doxirubicin-semiquinone radical. These enzymes include mitochondrial and cystolic NADPH dehydrogenates, xanthine oxidase, and nitric oxide synthases. This semiquinone metabolite can be re-oxidized to doxorubicin, although with the concurrent formation of reactive oxygen species (ROS) and hydrogen peroxide. It is the ROS generating through this pathway that contributes most to the doxorubicin-related adverse effects, particularly cardiotoxicity, rather than through doxorubicin semiquinone formation. Deglycosidation is a minor metabolic pathway, since it only accounts for 1 to 2% of doxorubicin metabolism. Under the catalysis of cytoplasmic NADPH quinone dehydrogenase, xanthine oxidase, NADPH-cytochrome P450 reductase, doxorubicin can either be reduced to doxorubicin deoxyaglycone or hydrolyzed to doxorubicin hydroxyaglycone.
Nonencapsulated doxorubicin is metabolized by NADPH-dependent aldoketoreductases to the hydrophilic 13-hydroxyl metabolite doxorubicinol, which exhibits antineoplastic activity and is the major metabolite; these reductases are present in most if not all cells, but particularly in erythrocytes, liver, and kidney. Although not clearly established, doxorubicinol also appears to be the moiety responsible for the cardiotoxic effects of the drug. Undetectable or low plasma concentrations (ie, 0.8-26.2 ng/mL) of doxorubicinol have been reported following iv administration of a single 10- to 50-mg/sq m dose of doxorubicin hydrochloride as a PEG-stabilized liposomal injection; it remains to be established whether such liposomally encapsulated anthracyclines are less cardiotoxic than conventional (nonencapsulated) drug, and the usual precautions for unencapsulated drug currently also should be observed for the liposomal preparation. Substantially reduced or absent plasma concentrations of the usual major metabolite of doxorubicin observed with the PEG-stabilized liposomal injection suggests that either the drug is not released appreciably from the liposomes as they circulate or that some doxorubicin may be released but that the rate of doxorubicinol elimination greatly exceeds the release rate; doxorubicin hydrochloride encapsulated in liposomes that have not been PEG-stabilized is metabolized to doxorubicinol.
Other metabolites, which are therapeutically inactive, include the poorly water-soluble aglycones, doxorubicinone (adriamycinone) and 7-deoxydoxorubicinone (17-deoxyadriamycinone), and conjugates. The aglycones are formed in microsomes by NADPH-dependent, cytochrome reductase-mediated cleavage of the amino sugar moiety. The enzymatic reduction of doxorubicin to 7-deoxyaglycones is important to the cytotoxic effect of the drug since it results in hydroxyl radicals that cause extensive cell damage and death. With nonencapsulated doxorubicin, more than 20% of the total drug in plasma is present as metabolites as soon as 5 minutes after a dose, 70% in 30 minutes, 75% in 4 hours, and 90% in 24 hours.
... At least 6 metabolites have been identified, the principal one being adriamycinol. This product results from redn of the keto group on C13 by an enzyme found in leukocytes and erythrocytes, and presumably in malignant tissues.
Doxorubicin is converted to doxorubicinol, to aglycones, and to other derivatives
For more Metabolism/Metabolites (Complete) data for DOXORUBICIN (6 total), please visit the HSDB record page.
Protein binding
The binding of doxorubicin and its major metabolite, doxorubicinol, to plasma proteins is 75% and is independent of plasma concentration of doxorubicin up to 1.1 µg/mL. Doxorubicin does not cross the blood-brain barrier. Plasma protein binding of doxorubicin hydrochloride liposome injection has not been determined.
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