Doxorubicin

Overview
Doxorubicin (trade name Adriamycin) or hydroxyldaunorubicin is a DNA-interacting drug widely used in chemotherapy. It is an anthracycline antibiotic and structurely closely related to daunomycin, and also intercalates DNA. It is commonly used in the treatment of a wide range of cancers.

The drug is administered by injection, and may be sold under the brand names Adriamycin PFS, Adriamycin RDF, or Rubex. Doxil is a liposome-encapsulated dosage form of doxorubicin made by Ben Venue Laboratories for Johnson & Johnson. The main benefits of this form are a reduction in cardiotoxicity.

History
The history of doxorubicin can be traced back to the 1950s, when an Italian research company, Farmitalia Research Laboratories, began an organized effort to find anticancer compounds from soil-based microbes. A soil sample was isolated from the area surrounding the Castel del Monte, a 13th century castle. A new strain of Streptomyces peucetius which produced a bright red pigment was isolated, and an antibiotic was produced from this bacterium that was found to have good activity against murine tumors. Since a group of French researchers discovered the same compound at about the same time, the two teams named the compound daunorubicin, combining the name Daunii, a pre-Roman tribe that occupied the area of Italy where the compound was isolated, with the French word for ruby, rubis, describing the color. Clinical trials began in the 1960s, and the drug saw success in treating acute leukemia and lymphoma. However, by 1967, it was recognized that daunorubicin could produce fatal cardiac toxicity.

Researchers at Farmitalia soon discovered that changes in biological activity could be made by minor changes in the structure of the compound. A strain of Streptomyces was mutated using N-nitroso-N-methyl urethane and this new strain produced a different, red-colored antibiotic. They named this new compound Adriamycin, after the Adriatic Sea, and the name was later changed to doxorubicin to conform to the established naming convention. Doxorubicin showed better activity than daunorubicin against murine tumors, and especially solid tumors. It also showed a relatively higher therapeutic index, yet the cardiotoxicity remained.

Doxorubicin and daunorubicin together can be thought of as prototype compounds for the anthracyclines. Subsequent research by many investigators throughout the world has led to many other anthracycline antibiotics, or analogs, and today, it is estimated that there are over 2,000 known analogs of doxorubicin. By 1991, 553 of them have been evaluated in the screening program at the National Cancer Institute (NCI).

Biosynthesis
Doxorubicin (DXR) is a 14-hydroxylated version of daunorubicin, the immediate precursor of DXR in its biosynthetic pathway. Daunorubicin is more abundantly found as a natural product because it is produced by a number of different wild type strains of streptomyces. In contrast, only one known non-wild type species, streptomyces peucetius subspecies cesius ATCC 27952, was initially found to be capable of producing the more widely used doxorubicin. This strain was created by Arcamone et. al in 1969 by mutating a strain producing daunorubicin, but not DXR, at least in detectable quantities. Subsequently, Hutchinson's group showed that under special environmental conditions, or by the introduction of genetic modifications, other strains of streptomyces can produce doxorubicin. His group has also cloned many of the genes required for DXR production, although not all of them have been fully characterized. In 1996, Strohl's group discovered, isolated and characterized dox A, the gene encoding the enzyme that converts daunorubicin into DXR. By 1999, they produced recombinant Dox A, a Cytochrome P450 oxidase, and found that it catalyzes multiple steps in DXR biosynthesis, including steps leading to daunorubicin. This was significant because it became clear that all daunorubicin producing strains have the necessary genes to produce DXR, the much more therapeutically important of the two. Hutchinson's group went on to develop methods to improve the yield of DXR, from the fermentation process used in its commercial production, not only by introducing Dox A encoding plasmids, but also by introducing mutations to deactivate enzymes that shunt DXR precursors to less useful products, for example baumycin-like glycosides. Some triple mutants, that also over-expressed Dox A, were able to double the yield of DXR. This is of more than academic interest because at that time DXR cost about $1.37 million per kg and current production in 1999 was 225 kg per annum. More efficient production techniques have brought the price down to $1.1 million per kg for the non-liposomal formulation. Although DXR can be produced semi-synthetically from daunorubicin, the process involves electrophilic bromination and multiple steps and the yield is poor. Since daunorubicin is produced by fermentation, it would be ideal if the bacteria could complete DXR synthesis more effectively.

Mechanism of action
The exact mechanism of action of doxorubicin is complex and still somewhat unclear, though it is thought to interact with DNA by intercalation. Doxorubicin is known to interact with DNA by intercalation and inhibition of macromolecular biosynthesis. This inhibits the progression of the enzyme topoisomerase II, which unwinds DNA for transcription. Doxorubicin stabilizes the topoisomerase II complex after it has broken the DNA chain for replication, preventing the DNA double helix from being resealed and thereby stopping the process of replication.

The planar aromatic chromophore portion of the molecule intercalates between two base pairs of the DNA, while the six-membered daunosamine sugar sits in the minor groove and interacts with flanking base pairs immediately adjacent to the intercalation site, as evidenced by several crystal structures.

Clinical use
Doxorubicin is commonly used to treat some leukemias, Hodgkin's lymphoma, as well as cancers of the bladder, breast, stomach, lung, ovaries, thyroid, soft tissue sarcoma, multiple myeloma, and others. Commonly used doxorubicin-containing regimens are CA (cyclophosphamide, Adriamycin), TAC (Taxotere, CA), ABVD (Adriamycin, Bleomycin, Vinblastine, Dacarbazine), CHOP (Cyclophosphamide, Adriamycin, Vincristine, Prednisone) and FAC (5-Fluorouracil, Adriamycin, Cyclophosphamide). Doxil is used primarily for the treatment of ovarian cancer where the disease has progressed or recurred after platinum-based chemotherapy, or for the treatment of AIDS-related Kaposi's sarcoma.

Experimental therapy
Combination therapy experiments with sirolimus (rapamycin) and doxorubicin have shown promise in treating Akt-positive lymphomas in mice.

Recent animal research coupling a murine monoclonal antibody with doxorubicin has created an immunoconjugate that was able to eliminate HIV-1 infection in mice. Current treatment with antiretroviral therapy (ART) still leaves pockets of HIV within the host. The immunoconjugate could potentially provide a complimentary treatment to ART to eradicate antigen-expressing T cells.

Side effects
Acute side-effects of doxorubicin can include nausea, vomiting, and heart arrhythmias. It can also cause neutropenia (a decrease in white blood cells), as well as complete alopecia (hair loss). When the cumulative dose of doxorubicin reaches 550 mg/m², the risks of developing cardiac side effects, including congestive heart failure, dilated cardiomyopathy, and death, dramatically increase. Doxorubicin cardiotoxicity is characterized by a dose-dependent decline in mitochondrial oxidative phosphorylation. Reactive oxygen species, generated by the interaction of doxorubicin with iron, can then damage the myocytes (heart cells), causing myofibrillar loss and cytoplasmic vacuolization. Additionally, some patients may develop Palmar plantar erythrodysesthesia, or, "Hand-Foot Syndrome," characterized by skin eruptions on the palms of the hand or soles of the feet, characterized by swelling, pain and erythema.

Due to these side effects and its red color, doxorubicin has earned the nickname "red devil."