thumb|Doxorubicin as an intercalating agent. Two doxorubicin molecules intercalated within DNA.
Anthracyclines are a class of drugs used in cancer chemotherapy that are extracted from Streptomyces peucetius bacterium. Their main adverse effect is cardiotoxicity, which considerably limits their usefulness. Use of anthracyclines has also been shown to be significantly associated with cycle 1 severe or febrile neutropenia. Other adverse effects include vomiting.
The drugs act mainly by intercalating with DNA and interfering with DNA metabolism and RNA production. Cytotoxicity is primarily due to inhibition of topoisomerase II after the enzyme induces a break in DNA, preventing religation of the break and leading to cell death. The basic structure of anthracyclines is that of a tetracyclic molecule with an anthraquinone backbone connected to a sugar moiety by a glycosidic linkage. When taken up by a cell the four ring structure intercalates between DNA bases pairs while the sugar sits within the minor groove and interacts with adjacent base pairs.
History
Daunorubicin is a red pigmented drug which was discovered in the early 1960s. It was isolated from a strain of Streptomyces peucetius by A. Di Marco and coworkers, working for Farmitalia Research Laboratories in Italy who called it daunomycin. About the same time Dubost and coworkers in France also discovered the compound and named it rubidomycin. Daunorubicin was adopted as the international name.
The first anthracyclines were so successful that thousands of analogues have been produced in attempts to find compounds with improved therapeutic applications. Only epirubicin and idarubicin have been adopted for worldwide use. Epirubicin has similar activity to doxorubicin, however has reduced cardiotoxic side effects. Idarubicin is a fat soluble variant of daunorubicin and is orally bioavailable.
Several groups of researchers focused on designing compounds that retained the polycyclic aromatic chromophore of the anthracyclines (favouring intercalation into DNA) and substituting the sugar residue with simple side chains. This led to the identification of the mitoxantrone which is classed as an anthracenedione compound and is used in the clinic for the management of various cancers. Disaccharide analogues have been shown to retain anticancer activity, and are being further investigated with respect to their mechanism of action.
Although it has been 50 years from the discovery of anthracyclines, and despite recent advances in the development of targeted therapies for cancers, around 32% of breast cancer patients, 57%-70% of elderly lymphoma patients and 50–60% of childhood cancer patients are treated with anthracyclines. Some cancers benefit from neoadjuvant anthracycline-based regimes, and these include triple negative breast cancers that do not respond well to targeted therapies due to the lack of available receptors that can be targeted. Compared to non-triple negative breast cancer patients, triple negative breast cancer patients have shown better response rate and higher pathological response rate with anthracycline use, an indicator used for predicting improved long-term outcomes.
Mechanism of action
thumb|Doxorubicin localisation to [[Cell nucleus|nuclei. Localisation of doxorubicin (red) in the nuclei of MCF-7<sub>cc10</sub> cells. Green fluorescence represents lysosome.|alt=|left]]
The anthracyclines have been widely studied for their interactions with cellular components and impact on cellular processes. This includes studies in cultured cells and in whole animal systems. A myriad of drug-cellular interactions have been documented in the scientific literature and these vary with respect to the properties of target cells, drug dose and drug intermediates produced. Since artefactual mechanisms of action can be observed, the following mechanisms which occur at clinically relevant drug concentrations are the most important.
DNA Intercalation
Anthracyclines are readily taken up by cells and localised to the nucleus. The chromophore moiety of anthracyclines has intercalating function and inserts in between the adjacent base pair of DNA. This topoisomerase-II-mediated DNA damage subsequently promotes growth arrest and recruits DNA repair machinery. When the repair process fails, the lesions initiate programmed cell death. In addition, the availability of cellular iron catalyses redox reactions and further generates ROS. The supply of extracellular formaldehyde using formaldehyde-releasing prodrugs can promote covalent DNA adduct formation. Such adducts have been shown to block GpC specific transcription factors and induce apoptotic responses.
Clinical implications
Results from a recent meta-analysis provide evidence that breast cancer patients with either duplication of centromere 17 or aberrations in TOP2A, the gene coding for topoisomerase-IIα, benefit from adjuvant chemotherapy that incorporates anthracyclines. This does not include subgroups of patients that harbour amplification of HER2. The observations from this study also allow patients to be identified where anthracyclines might be safely omitted from treatment strategies.
Anthracycline-mediated cardiotoxicity is dose-dependent and cumulative, with the damage imposed to heart occurring upon the very first dose and then accumulating with each anthracycline cycle. There are four types of anthracycline-associated cardiotoxicity that have been described.
{| class="wikitable"
|+Anthracycline-mediated cardiotoxicity progression and symptoms
!Types of cardiotoxicity
!Time to presentation
!Symptoms
|-
|Acute
|During and immediately after drug administration
|Vasodilation, hypotension, transient cardiac rhythm disturbances
|-
|Subchronic
|1–3 days post-drug administration
|Pericarditis-myocarditis
|-
|Early chronic
|Less than 1 year after completing anthracycline treatment
|Dilated cardiomyopathy, restrictive cardiomyopathy (uncommon), left ventricular contractile dysfunction, congestive heart failure
|-
|Delayed/late onset chronic
|More than 1 year after completing anthracycline treatment
|Restrictive cardiomyopathy, dilated cardiomyopathy, congestive heart failure
|}
In the clinic, a maximum recommended cumulative dose is set for anthracyclines to prevent the development of congestive heart failure. As an example, the incidence of congestive heart failure is 4.7%, 26% and 48% respectively when patients received doxorubicin at 400 mg/m<sup>2</sup>, 550 mg/m<sup>2</sup> and 700 mg/m<sup>2</sup>.
In order to reduce the impact of cardiac injury in response to anthracyclines, a few cardioprotective strategies have been explored. Liposomal formulations of anthracyclines (discussed below) have been developed and used to reduce cardiac damage. Other novel anthracycline analogues such as epirubicin and idarubicin also provide options to reduce adverse cardiac events; these analogues have failed to show superior anti-cancer activity to the parent compounds. Extravasation causes serious complications to surrounding tissues with the symptoms of tissue necrosis and skin ulceration. Studies of the cardioprotective nature of dexrazoxane, provide evidence that it can prevent heart damage without interfering with the anti-tumour effects of anthracycline treatment. Patients given dexrazoxane with their anthracycline treatment had their risk of heart failure reduced compared to those treated with anthracyclines without dexrazoxane. There was no effect on survival though.
Radiolabelled doxorubicin has been utilised as a breast cancer lesion imaging agent in a pilot study. This radiochemical, <sup>99m</sup>Tc-doxorubicin, localised to mammary tumour lesions in female patients, and is a potential radiopharmaceutical for imaging of breast tumours.
In some cases, anthracyclines may be ineffective due to the development of drug resistance. It can either be primary resistance (insensitive response to initial therapy) or acquired resistance (present after demonstrating complete or partial response to treatment). Resistance to anthracyclines involves many factors, but it is often related to overexpression of the transmembrane drug efflux protein P-glycoprotein (P-gp) or multidrug resistance protein 1 (MRP1), which removes anthracyclines from cancer cells.|size2=140px|direction=vertical
Liposomes are spherical shape, phospholipid vesicles that can be formed with one or more lipid bilayers with phospholipids or cholesterols. The ability of liposomes to encapsulate both hydrophobic and hydrophilic drug compounds allowed liposomes to be an efficient drug delivery systems (DDS) to deliver a range of drugs in these nano-carriers. Doxorubicin is encapsulated in a nano-carrier known as Stealth or sterically stabilised liposomes, consisting of unilamellar liposomes coated with hydrophilic polymer polyethylene glycol (PEG) that is covalently linked to liposome phospholipids. The PEG coating serves as a barrier from opsonisation, rapid clearance while the drug is stably retained inside the nano-carriers via an ammonium sulphate chemical gradient. A major advantage of using nano-carriers as a drug delivery system is the ability of the nano-carriers to utilise the leaky vasculature of tumours and their impaired lymphatic drainage via the EPR effect.
The maximum plasma concentration of free doxorubicin after Doxil administration is substantially lower compared to conventional doxorubicin, providing an explanation for its low cardiotoxicity profile.
{| class="wikitable"
|+Characteristic comparison between Doxil and Myocet
|
!Doxil
!Myocet
!References
|-
!scope=row|Composition of liposomes
|PEG-phospholipid
Phospholipid
Cholesterol
|Phospholipid
Cholesterol
|
|-
!scope=row|Size
|80 nm – 100 nm
|150 nm - 250 nm
|
|-
!scope=row|Drug loading method
|Ammonium salt gradient
|Citric acid gradient
|
|-
!scope=row|Clinical indication
|AIDS-related Kaposi's sarcoma, recurrent ovarian cancer and metastatic breast cancer
|Metastatic breast cancer
| As they act as antibiotics anthracyclines can reduce the effectiveness of live culture treatments such as Bacillus Calmette-Guerin therapy for bladder cancer. As they act as myelosuppressors anthracyclines can reduce the effectiveness of vaccines by inhibiting the immune system.
Several interactions are of particular clinical importance. Though dexrazoxane can be used to mitigate cardiotoxicity or extravasation damage of anthracyclines it also may reduce their effectiveness and the recommendation is not to start dexrazoxane treatment upon initial anthracycline treatment. Trastuzumab (a HER2 antibody used to treat breast cancer) may enhance the cardiotoxicity of anthracyclines although the interaction can be minimised by implementing a time interval between anthracycline and trastuzumab administration. Taxanes (except docetaxel) may decrease anthracycline metabolism, increasing serum concentrations of anthracyclines. The recommendation is to treat with anthracyclines first if combination treatment with taxanes is required.