This liposome-encapsulated form is also approved by the FDA for treatment of ovarian cancer and multiple myeloma.A non-pegylated liposomal doxorubicin, called Myocet, is approved in the European Union and in Canada for the treatment of metastatic breast cancer in combination with cyclophosphamide, but it has not been approved by the FDA for use in the United States. Unlike Doxil, the Myocet liposome does not have a polyethylene glycol coating, and therefore does not result in the same rate of PPE. The minimization of this side effect may allow for one-for-one (1:1) substitution with doxorubicin in the same treatment regimen, thereby improving safety with no loss of efficacy. Like Doxil, the liposomal encapsulation of the doxorubicin limits the cardiotoxicity. In theory, by limiting the cardiotoxicity of doxorubicin through liposomal encapsulation, it can be used safely in concurrent combination with other cardiotoxic chemotherapy drugs, such as trastuzumab. There is an FDA black box warning that trastuzumab cannot be used in concurrent combination with doxorubicin, only in sequential combination. Though concurrent combination of trastuzumab and doxorubicin in clinical studies found superior tumor response, the combination resulted in unacceptable cardiotoxicity, including risk of cardiac failure manifesting as congestive heart failure (CHF). Published phase II study results have shown that Myocet, trastuzumab, and paclitaxel can safely be used concurrently without the cardiac risk, as measured by reduction in LVEF function, while still achieving superior tumor response. This finding is the basis for the ongoing phase III trial for FDA approval.
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, Hutchinsons group showed that under special environmental conditions, or by the introduction of genetic modifications, other strains of Streptomyces can produce doxorubicin. His group also cloned many of the genes required for DXR production, although not all of them have been fully characterized. In 1996, Strohls 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. Hutchinsons 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.