Cancer is a leading cause of morbidity and mortality worldwide. It is estimated that there will be 13.1 million deaths due to cancer in 2030. Anthracyclines, especially doxorubicin, have been the mainstay of cancer therapy since long. Despite its broad-spectrum antineoplastic activity [Table 1], adverse events [Table 2], particularly cardiotoxicity, has limited the use of conventional doxorubicin in clinical practice. This was especially so in patients with advanced disease requiring dose escalation.
Doxorubicin hydrochloride (HCl) liposomal injection was the first liposomal encapsulated anticancer drug to receive clinical approval and has activity against a number of malignancies including solid tumors, transplantable leukemias, and lymphomas.
Previous clinical studies have focused on the lower toxicity and better tolerability of liposomal anticancer drugs. However, in recent years, research has focused on developing novel liposomal formulations. This review is aimed at providing an overview of doxorubicin in cancer therapy.
HISTORY OF DOXORUBICIN
The quest for anticancer compounds from soil-based microorganisms began in the 1950s. 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 mouse tumors. The new compound was named daunorubicin and was used successfully in the treatment of acute leukemia and lymphoma. However, by 1967, it was recognized that daunorubicin could produce fatal cardiac toxicity.
Researches made genetic modifications to the Streptomyces spp. to produce a compound, Adriamycin, which was later called doxorubicin. Though doxorubicin had a higher therapeutic index cardiotoxicity continued to be a major problem. These compounds have been prototypes for subsequent research and today there are approximately more than 2000 known analogs of doxorubicin.
MECHANISM OF ACTION OF DOXORUBICIN
The exact mechanism of action of doxorubicin is complex and still unclear. Doxorubicin interacts with the DNA by intercalation thus, inhibiting macromolecular biosynthesis. This further inhibits the progression of the enzyme topoisomerase II, and relaxes supercoils in 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 [Figure 1]. Another mechanism of doxorubicin HCl is its ability to generate free radicals that induce DNA and cell membrane damage.
DOXORUBICIN IN CANCER THERAPY
Doxorubicin is most commonly used to treat cancers of the bladder, breast, stomach, lung, ovaries, thyroid, soft tissue sarcoma, multiple myeloma, and Hodgkin's lymphoma. The commonly used doxorubicin-containing regimens may include Adriamycin, cyclophosphamide (AC), Taxotere, AC, Adriamycin, bleomycin, vinblastine, dacarbazine, bleomycin, etoposide, AC, vincristine, procarbazine, and prednisone, cyclophosphamide, Adriamycin, vincristine, prednisone and 5-fluorouracil, AC. Doxil is mainly used for the treatment of ovarian cancer where it has progressed or recurred after platinum-based chemotherapy, or for the treatment of AIDS-related Kaposi's sarcoma.
CARDIOTOXICITY ASSOCIATED WITH CONVENTIONAL DOXORUBICIN
The cardiotoxicity associated with conventional doxorubicin is broadly classified as either acute or chronic. It is a major concern during therapy as it may be dose-limiting. Risk factors that may increase the occurrence of cardiotoxicity include previous or current heart disease, extremes in age, race, exposure to irradiation in the mediastinal region and the cumulative dose of doxorubicin received. Patients with acute doxorubicin-induced cardiotoxicity usually present with rhythm disturbances, abnormal electrocardiographic changes, and acute, but reversible, reductions in left-ventricular ejection fraction. In general, these symptoms occur within 24 h of infusion, are self-limiting, and do not appear to increase the risk of future cardiac events. Cardiotoxicity, a consequence of conventional doxorubicin treatment manifests as a chronic complication, which ultimately results in congestive cardiomyopathy. Such toxicity has been shown to correlate with peak plasma doxorubicin concentrations, as well as with the lifetime cumulative dose administered. Historically, cardiomyopathy has rarely been observed with cumulative doses of conventional doxorubicin below 450 mg/m2; however, studies suggest that doxorubicin-induced congestive heart failure (CHF) may occur at lower-doses and with greater frequency than previously noted. Von Hoff et al. reported incidence of conventional doxorubicin-induced CHF of approximately 3% at cumulative dose of 400 mg/m2, increasing to 7% at 550 mg/m2 and to 18% at 700 mg/m2.
RATIONALE FOR THE DEVELOPMENT OF NOVEL DOXORUBICIN FORMULATIONS
Several efforts have been made to improve the toxicity profile of conventional anthracyclines, including the development of novel anthracycline analogs, the use of low-dose, prolonged, continuous infusion schedules; co-administration with the cardioprotective agent dexrazoxane; and the use of liposomal encapsulation technology.
The therapy-limiting toxicity for this drug is cardiomyopathy, which may lead to CHF and death. An approach to ameliorating doxorubicin-related toxicity is to use drug carriers, which engender a change in the pharmacological distribution of the drug, resulting in reduced drug levels in the heart. Examples of these carrier systems include lipid-based (liposome) formulations that effect a beneficial change in doxorubicin. Liposomal encapsulation represents the most successful strategy to date for increasing the therapeutic index of conventional anthracyclines. Liposomal drug-delivery systems have been studied extensively to increase the therapeutic index of chemotherapy. The development of liposomal anthracyclines was based on the rationale that liposomes cannot escape the vascular space in areas with tight capillary junctions (heart muscle) but can leave the circulation in tissues and organs lined with cells that are not tightly joined (tumor cells). Thus, liposomal encapsulation results in the preferential concentration of anthracyclines in tumor tissue, while limiting exposure in those sites most frequently associated with conventional anthracycline toxicity, such as the myocardium.
LIPOSOMAL FORMULATIONS OF DOXORUBICIN
Due to the side-effects of conventional doxorubicin therapy, a need for development of liposomal formulation with equal efficacy and less side-effects arose. One of the main advantages of using liposomes as a delivery system is that the phospholipids used to form these vesicles are extracted from natural sources such as egg yolks or soybean and hence they are safe in the body. In addition, the degree of saturation of phospholipid bilayer can be modified to alter the drug release rate.
Liposomes, composed of natural phospholipids mixed with varying amounts of cholesterol are removed from circulation by the reticuloendothelial system (RES) within a few minutes to a few hours, subsequent to the acquisition of opsonins from plasma. Because of this short circulation half-life, the use of conventional liposomes has limited clinical applications.
PEGYLATED LIPOSOMAL FORMULATIONS OF DOXORUBICIN
The fact that some polymers, such as polyethylene glycol (PEG), ganglioside, and cerebroside sulfate are able to inhibit opsonization of the liposomes by plasma proteins and increase the half-life of liposomal drugs has renewed activity in the area of liposomal drug-delivery systems. Prolonged circulation of liposomes has been linked to better therapeutic efficacy of liposomal anthracyclines, possibly related to increased accumulation of drug loaded liposomes in tumor tissue.
A further advancement in this line of treatment was the pegylated-liposomal doxorubicin (such as Doxil®/Caelyx®), a unique form of liposomal doxorubicin, in which the liposomes were coated with PEG in a process known as pegylation. This prevents drug uptake by the RES, thereby prolonging the circulation time (half-life: 3-4 days vs. 30 h for conventional doxorubicin) beyond that of conventional liposomes and enabling the drug to remain encapsulated until it reaches the tumor site.
Doxil® is approved by the Food and Drug Administration (FDA) for treatment of ovarian cancer after failure of first line platinum-based chemotherapy and multiple myeloma. Outside the United States, Doxil® is known as Caelyx® and is marketed by Janssen. However, there is some evidence that raises doubts on the benefits of pegylation in cancer treatment. PEG is a large molecule and its presence on the liposomal surface may decrease the interactions of liposomes with cells and impede the entry of liposomes into the tumor tissue. This may possibly reduce the accumulation of liposomal drugs in the tumor tissue. A study assessing the accumulation of liposomal doxorubicin in murine Lewis lung carcinoma showed that benefits with PEG liposomal doxorubicin, such as increased blood levels and circulation time may be of little advantage when compared to maximizing drug accumulation in tumors.
However, pegylated-liposomal doxorubicins cause dose-limiting hand-foot syndrome (HFS) (palmar-plantar erythrodysesthesia) characterized by skin eruptions on the palms of the hand and/or soles of the feet, leading to interruption in therapy by at least 2 weeks and decrease in subsequent dosage by 25% (Doxil Product Information Leaflet). The incidence of HFS has been observed in approximately 50% of patients dosed at 50 mg/m2(Doxil Product Information leaflet).
Though Caelyx®/Doxil®(pegylated liposomal doxorubicin) has high affinity for the skin and a very long circulation time, a major limitation is dose-limiting HFS (palmar-plantar erythrodysesthesia).
NONPEGYLATED LIPOSOMAL FORMULATIONS OF DOXORUBICIN
There remains a need for stable, long circulating liposomes that do not cause such deleterious effects such as the HFS. The new variant of liposomal doxorubicin, nonpegylated liposomal doxorubicin (NPLD), has a unique drug-delivery system, and provides a breakthrough in cancer therapy by offering the benefits of pegylated-liposomal doxorubicin without its major side-effects such as HFS.
Nonpegylated liposomal doxorubicin injection provides a better safety profile than not only conventional doxorubicin, but also Doxil®/Caelyx®. NPLD not only reduces the cardiac toxicity associated with doxorubicin, but also the dose-limiting toxicity associated with the use of Doxil®/Caelyx®, such as HFS. This is achieved by a combination of (1) specific composition and (2) a unique manufacturing process (both which have been patented) of the NPLD's liposome, which gives it the desired physicochemical properties.
The NPLDs have an increased circulation time and less cardiotoxicity as compared with conventional doxorubicin. Since NPLDs do not have PEG coating, they are not associated with the painful HFS, which is a dose-limiting adverse event with PEG-doxorubicin.
Myocet® is a NPLD manufactured by Enzon Pharmaceuticals for Cephalon in Europe and for Sopherion Therapeutics in the United States and Canada. Myocet® is approved in Europe and Canada for treatment of metastatic breast cancer in combination with cyclophosphamide. However, it is under the process for the US FDA approval. Three Phase III trials in breast cancer evaluated Myocet® in comparison with standard doxorubicin and epirubicin. Compared to standard doxorubicin, Myocet® demonstrated less cardiotoxicity with no loss of efficacy. Equal doses of epirubicin and Myocet® were as effective with no significant difference in cardiotoxicity. Another limitation of Myocet® is related to its administration which is presented as a three-vial system; Myocet doxorubicin HCl, Myocet liposomes and Myocet buffer. In addition, a small amount of 0.9% sodium chloride for injection is needed, which is not provided in the package. The constituted product is a liposome-encapsulated doxorubicin-citrate complex. Further, the high cost of Myocet® has been a hindrance in its widespread use. This has been a limitation in its robust assessment within the treatment pathway for advanced cancers.
Nudoxa®(NPLD) with its unique drug-delivery system is a breakthrough in cancer therapy as it offers the benefit of pegylated liposomal doxorubicin without its major side-effects like HFS. Further, it decreased the toxicity and other adverse events such as nausea, vomiting, and alopecia. Nudoxa® was jointly developed by Zydus and Bharat Serums and Vaccines Pvt. Limited Nudoxa® is being used as a chemotherapy drug to treat various cancers, such as breast cancer, ovarian cancer, and AIDS-related Kaposi sarcoma. The formulation uses distearoyl-phosphatidylcholine, a phospholipid with high phase transition temperature, which increases the retention of the liposomal content. This allows the doxorubicin HCl liposomes to circulate for prolonged periods in the bloodstream. Liposomes are small enough (average diameter of approximately 100 nm) to pass intact (extravasate) through the defective blood vessels present in the tumors. The liposomal covering helps the drug from escaping into the body's immune system thereby enabling the drug to reach malignant cells. This results in reduction of harmful side-effects owing to minimized distribution of the drug to nontargeted tissues. The preclinical studies were aimed at comparing Nudoxa® with PEG liposomal doxorubicin and conventional doxorubicin (data on file). Nudoxa® has demonstrated favorable toxicity profile in animal models. Further, it offers an advantage over pegylated liposomal doxorubicin due to the absence of HFS. Maximum tolerated dose for Nudoxa® has been established at 70 mg/m2 in Phase I clinical trials (data on file).
Nudoxa® was launched in the Indian market in 2008 and is being used as a chemotherapy drug to treat various cancers, particularly, breast cancer, ovarian cancer and AIDS-related Kaposi sarcoma. A Phase II/III open label multicentric randomized trial determined the safety and efficacy of Nudoxa® at two different dose levels as compared to doxorubicin in patients with metastatic breast cancer. The trial reported that Nudoxa® at 70 mg/m2 was as efficacious as conventional doxorubicin with a better safety profile (CTRI/2009/091/1000795).
OTHER NOVEL FORMULATIONS OF DOXORUBICIN
Stealth liposomes have gained an important place in cancer chemotherapy, but still these liposomes are modulated in terms of their selectivity for tumors by attachment of ligands. Antibody coated liposome (immunoliposomes) are extensively studied where either an antibody is attached directly to liposome phospholipid head group or to the terminus of PEG polymer. Temperature sensitive liposomes are also fabricated for tumor targeting. Similarly, acid triggered, enzyme triggered and light triggered release are being studied for delivery of doxorubicin. Other case of active targeting are sulfatide-mediated liposome and folate receptor targeting. Block co-polymers of poly (ethylene oxide)-poly (propylene oxide) block copolymers (pluronics) have also being developed for doxorubicin. Hydroxyapatite implants containing doxorubicin have been developed and tested in in vivo model for osteogenic sarcoma. Thermosensitive poly (organophosphazenes) hydrogels containing doxorubicin have also been studied. The release of loaded doxorubicin from the polymer hydrogel was significantly sustained over 20 days. Oral delivery of microgels composed of pluronics was found to be active against Caco-2 cells. Biological particles like resealed erythrocytes and bacterial ghosts are being explored for the delivery of doxorubicin are also proving to be quite effective. Nanoparticles prepared using polybutylcyanoacrylate as delivery system for doxorubicin to target liver cancer have been studied. The findings suggested that drug was released slowly in liver during detection period in treatment groups. Nanoparticles in range of 100-150 nm diameter range had the best liver targeting characteristics. It also decreased doxorubicin distribution in other tissues such as heart, kidney, and lung. Poly (butylcyanoacrylate) nanoparticles coated with polysorbate 80 considerably enhance the antitumor effect of doxorubicin against an intracranial glioblastoma in rats.
Doxorubicin-loaded polymer-lipid hybrid nanoparticles were formulated and evaluated in a murine solid tumor model. A new bioadhesive drug-delivery system, poly (d, l-lactide-co-glycolide/montmorillonite nanoparticles were developed for oral delivery of paclitaxel. Such a novel formulation is expected to possess extended residence time in the gastrointestinal tract, which promotes oral delivery of paclitaxel.
Among the novel formulations, only the liposomal doxorubicin formulations have been extensively studied and explored in clinical trials in patients with different stages of cancer.
Conventional doxorubicin has been a mainstay of treatment for breast and ovarian cancers. Although the use of conventional doxorubicin has been somewhat limited by its adverse effects, recent efforts have markedly improved its safety and tolerability. Until date, the most successful strategy for improving the therapeutic index of conventional doxorubicin formulations has been liposomal encapsulation, which results in preferential accumulation of drug within the tumor site to maximize efficacy and minimize toxicity. Pegylated liposomal doxorubicin has demonstrated efficacy in ovarian cancer patients who have failed platinum and paclitaxel therapies, as well as in metastatic breast cancer. However, dose-limiting HFS limits its usage. Nudoxa® is a NPLD, in which doxorubicin is confined in nonpegylated liposomes. Safety and efficacy of Nudoxa® has been established in several preclinical studies (data on file). Currently Nudoxa® is being studied in clinical trials to establish its safety and efficacy in different types of cancers. The early results show that Nudoxa® is as efficacious as conventional doxorubicin with a better safety profile. This may present an improved treatment regimen for late stage cancer patients.
With established efficacy and safety profiles, future studies will hopefully be directed toward estimating the costs of treatment with the novel liposomal doxorubicin formulations in order to assess their widespread use and robustness in treating patients with cancer.
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Conflict of Interest: None declared.