Liposome encapsulation of antineoplastic drugs entered clinical testing in the late 1980s. As carriers for a variety of agents, liposomes can allow successful delivery of agents that may be subject to rapid degradation in
ABSTRACT: Liposome encapsulation of antineoplastic drugs entered clinicaltesting in the late 1980s. As carriers for a variety of agents, liposomes canallow successful delivery of agents that may be subject to rapid degradation inthe serum and can modify the toxicity profile. In general, liposomes havedemonstrated an ability to attenuate toxicities by their differentpharmacokinetic profile and pattern of distribution. Differences in theconstitution of the liposome can greatly affect the pharmacokinetic profileresulting in different patterns of toxicity. Characteristics such as size,charge, composition, and integrity can affect performance of the liposome.Liposome encapsulation of doxorubicin has been shown to reduce cardiac toxicity.Preliminary data suggest that encapsulation of paclitaxel can greatly modifyneurotoxicity without the need for cremephor. [ONCOLOGY 15(Suppl 7):44-48, 2001]
The majority of anticancer drugsare nonspecific, cytotoxic agentsthat damage tumor cells as well as normal healthy tissue. Given their narrowtherapeutic index, antineoplastic drugs have the potential for serious sideeffects. A variety of drug-delivery systems are currently being used in aneffort to make anticancer agents more efficient and less toxic. Liposomalencapsulation of chemotherapeutic agents represents one method of achieving thisgoal.
First described by Bangham more than 30 years ago, liposomeshave undergone significant refinements over the decades to evolve into importantmodern-day drug carriers. Liposomes are spherical vesicles comprised of an outerphospholipid membrane with an internal aqueous compartment. Water-soluble drugscan be contained in the aqueous compartment while hydrophobic drugs incorporatewithin the lipid bilayer.
Liposomes are ideal candidates for drug-delivery systems. As aresult of their similarity to biological membranes, they are safe,biocompatible, and biodegradable. Liposomes are extremely versatilemacromolecules. By manipulating certain physical parameters such as phospholipidcomposition, size, or membrane characteristics, liposomes can be engineered toefficiently encapsulate and effectively transport a variety of drugs. Smallchanges in these parameters can also have profound effects on thepharmacokinetic and pharmacodynamic profiles of a drug.[2-9]
The advancement of the liposomal drug-delivery system in theoncology field was predicated on preclinical and clinical data demonstrating thebenefits of liposomal encapsulation of chemotherapeutic agents. Some of theliposomal preparations significantly increase the half-life of the incorporatedagent vs that of the free drug.[10-13]
Shielding the free drug within a liposome protects it fromplasma protein interaction and metabolic degradation. As a result,pharmacokinetic parameters of the free drug are altered by liposomalencapsulation. Indeed, chemotherapeutic agents incorporated into liposomesexhibit longer circulation lifetimes with a greater area under the curve, lowerrate of clearance, and smaller volume of distribution as compared with that ofthe free drugs.[13-19] The narrow therapeutic index of many small-moleculechemotherapeutic drugs is, in part, a consequence of their large volume ofdistribution. Incorporation into liposomes significantly reduces the volume ofdistribution, thereby decreasing the toxicity to normal tissue and increasingthe amount of drug that can be effectively delivered to the tumor.[20,21]
The anthracycline doxorubicin is one of the most importantcytotoxic agents used in anticancer therapy. It has significant clinicalactivity in hematologic malignancies, such as lymphomas and leukemias, as wellas in numerous solid tumors. However, our ability to achieve maximum clinicalefficacy with doxorubicin is limited by the development of irreversiblecardiotoxicity and multidrug resistance (MDR).
Myelosuppression is the dose-limiting toxicity, butcardiotoxicity remains the therapy-limiting toxicity. In order to avoid thiscomplication, therapy is generally discontinued when the cumulative dose reaches450-500 mg/m2, at which point the risk of cardiotoxicity is significant.[22-24]Liposomes have been successfully employed to circumvent some of the limitationsof free doxorubicin. Liposomal encapsulation of doxorubicin significantly altersits pharmacokinetic profile, attenuates its toxicity patterns in clinicalsettings, and demonstrates in preclinical trials an ability to reverse multidrugresistance.[25-28]
Alters Pharmacokinetic Profile
Numerous phase I studies have clearly demonstrated significantlyhigher areas under the curve for liposomal-encapsulated doxorubicin (LED) thanfor similar intravenous bolus doses of free doxorubicin.[29-32] Free doxorubicinreaches a peak concentration within minutes after administration and has aterminal elimination half-life of 30 hours. Depending on the specific liposomalformulation, doxorubicin-containing liposomes can increase the area under thecurve by 20 to 600 times.[31,32] Therefore, the pharmacokinetic pattern ofliposomal-encapsulated doxorubicin more closely mimics the prolonged infusionregimen of doxorubicin, which has demonstrated less systemic toxicity, mostnotably diminished cardiotoxicity and gastrointestinal toxicity.
Attenuates Toxicity Patterns
Additionally, liposomal-encapsulated doxorubicin exhibits a muchsmaller volume of distribution as compared with free doxorubicin. The estimatedvolume of distribution for free doxorubicin is approximately 25 L/kg, or 1,875liters for a 70-kg person, reflecting significant tissue uptake.[34,35] Incontrast, liposomal preparations can reduce the volume of distribution by 10- to60-fold.[10-12,31,32] The relatively large size of liposomes probably accountsfor the marked reduction in the volume of distribution.
These large macromolecules are usually unable to traverse thesmall endothelial gaps of normal tissue, especially cardiac endothelialcells.[21,36] Indeed, a large body of clinical data supports findings thatcancer patients treated with liposomal-encapsulated doxorubicin have asignificant reduction in cardiac toxicity.[37-41] Diminished hematologictoxicity (myelosuppression), gastrointestinal toxicity (nausea/vomiting),mucositis, and venous sclerosis have also been noted with liposomal-encapsulateddoxorubicin.
Shown in Preclinical Trials to Reverse Multidrug Resistance
Multidrug resistance is an important impediment that isdifficult to circumvent in the treatment of cancer. Overexpression ofP-glycoprotein (Pgp170) in neoplastic cells can lead to marked resistance todoxorubicin. Preclinical studies have demonstrated liposomal modulation ofMDR.[25-28] In one study, liposomal-encapsulated doxorubicin was shown toprevent effective multidrug resistance by binding to the P-glycoprotein plasmamembrane pump expressed by the multidrug resistant genes. This may helpexplain why liposomal-encapsulated doxorubicin has been effective in thetreatment of various chemotherapy-refractory cancers.[42-44]
Extensively Studied in Treatmentof Advanced Breast Cancer
Liposomal doxorubicin is steadily proving to be an importantantitumor agent. It has been extensively studied in the treatment of advancedbreast cancer, ovarian cancer, and Kaposi’s sarcoma.
The first phase II study of liposomal-encapsulated doxorubicinin advanced breast cancer included 20 patients who had not receivedanthracycline treatment within the past year. Sixteen of the 20 patientsreceived doxorubicin doses of 75 mg/m2 every 3 weeks, while the four remaining patients, all of whom hadundergone extensive radiotherapy, received either 45 or 60 mg/m2.
Overall, five patients demonstrated a complete response in theirindex lesion, while four others demonstrated objective disease regression. Themean duration of response was 7 months.
Most patients had some degree of myelosuppression, but the meannadir leukocyte count for all the cycleswas only 3,740/mL, and no patient developed sepsis or infection. Other toxiceffects included two episodes of grade 4 nausea/vomiting, two episodes of mildstomatitis, and complete alopecia in all patients.
Cardiotoxicity was also assessed. Two patients had leftventricular ejection fraction decreases (13% and 17%), and endomyocardialbiopsies performed on five patients who received cumulative doses of more than500 mg/m2 revealed four grade 0 and one episode of grade 1 structural changes.These data demonstrate the clinical activity of liposomal-encapsulateddoxorubicin against breast cancer, and suggest that liposomal-encapsulateddoxorubicin causes less cardiotoxicity, myelosuppression, and gastrointestinaltoxicity than equivalent doses of free doxorubicin.
Subsequent trials in advanced breast cancer have demonstratedgood response rates and a favorable toxicity profile, especially reducedcardiotoxicity.[39,45] A recent phase III trial was reported comparingcyclophosphamide (Cytoxan, Neosar) and either TLC D-99 or standard doxorubicinas first-line treatment of metastatic breast cancer. Response rates were 43%in both arms with a median survival of 21.2 months for the TLC D-99 patients and16.4 months for those receiving standard doxorubicin. As expected, those treatedwith TLC D-99 had significantly less cardiotoxicity and grade 4myelosuppression.
Clinical Response Against Refractory Ovarian Cancer
Favorable results have also been demonstrated in patients withrefractory ovarian cancer. A phase II trial of liposomal doxorubicin wasconducted in 35 women with ovarian cancer refractory to platinum- and paclitaxel(Taxol)-based regimens. Patients received 50 mg/m2 IV every 3 weeks. Nineclinical responses were observed (one complete response and eight partialresponses) in 35 patients (25.7% response rate). The median progression-freesurvival was 5.7 months with an overall survival of 1.5 to 24 + months (median:11 months).
Four patients developed fever and grade 3 neutropenia, while tendeveloped grade 3 hand-foot syndrome. No cases of alopecia, cardiotoxicity,phlebitis, or hepatic dysfunction were observed. This result suggests thatliposomal doxorubicin might be useful as second-line therapy for this disease,and the lack of moderate-to-severe myelosuppression indicates that it canpotentially be integrated into combined drug modalities for ovarian cancerwithout requiring dose attenuation.
Favorable Response Among Kaposi’s Sarcoma Patients
Patients with Kaposi’s sarcoma also respond favorably to thisformulation. A number of phase II and III trials have shown liposomaldoxorubicin to be an effective agent in the treatment of AIDS-associated Kaposi’ssarcoma. Single-agent therapy produced a significantly better overall responserate with much less toxicity and greater patient compliance vs a multidrugregimen.[44,48-50] Future trials, testing the formulation in combination withother active agents, are planned.
Paclitaxel is an active antineoplastic agent that was derivedfrom the bark extract of the Pacific yew (Taxus brevifolia). It promotesassembly and enhances stability of microtubules, the components of theintracellular skeleton, which modulate mitosis as well as other biologicalfunctions such as protein secretion, intracellular transport, and cellmotility.[51,52] By altering the dynamic nature of microtubule assembly anddisassembly, which is critical for entry into and execution of the mitotic cellcycle, paclitaxel leads to functional mitotic arrest of affected cells in the G2and M phases.[53,54] Numerous clinical studies have demonstrated the activity ofpaclitaxel in a variety of malignancies, including breast, ovarian, lung, andhead and neck tumors.[55-59]
Despite its broad and promising antitumor profile, paclitaxel isassociated with many clinically relevant toxic side effects. Peripheralneuropathy is a significant clinical toxicity. Hypersensitivity reactionscharacterized by dyspnea, hypotension, angioedema, and generalized urticaria aswell as cardiac arrhythmias may occur. Other problematic side effects includemyalgia and arthralgia, alopecia, nausea, vomiting, diarrhea, mucositis, andphlebitis.[60-62]
Given the potent and extensive antitumor effects of paclitaxel,attenuation of its systemic toxicity would be a significant accomplishment inthe field of oncology. Because liposomes have successfully reduced the toxicityprofile of various other chemotherapeutic agents while maintaining and sometimesimproving efficacy, the application of the liposomal vector to paclitaxel isintriguing.
Modulates Multidrug Resistance
A liposome-encapsulated paclitaxel preparation has beendeveloped that overcomes the stability and solubility problems associated withconventional paclitaxel. Preclinical evaluations demonstrated thatliposome-encapsulated paclitaxel modulates multidrug resistance in humanpromyelocytic leukemia HL-60/VCR cells by ninefold, and in human ovarian SKVLBcancer cells by eightfold compared with conventional paclitaxel.
Intracellular paclitaxel accumulation in these cell lines wasenhanced by threefold to fourfold when presented in liposomes.Liposome-encapsulated paclitaxel also showed a significant reduction in theefflux rate of paclitaxel from the cells as compared with that of conventionalpaclitaxel.
Significantly Less Toxic
Another preclinical study was designed to evaluate thepharmacokinetics, tissue distribution, toxicity, and therapeutic efficacy ofliposome-encapsulated paclitaxel in comparison to conventional paclitaxel. Innormal mice, liposome-encapsulated paclitaxel was much less toxic than standardpaclitaxel with comparable antitumor activity. The area under the curve wastwofold higher and the elimination half-life was two times longer withliposome-encapsulated paclitaxel than with conventional paclitaxel.
As expected, conventional paclitaxel displayed nonlinearpharmacokinetics with a disproportionate increase in area under the curvecompared to the dose administered. Interestingly, at the dose levels studied,liposome-encapsulated paclitaxel demonstrated linear kinetics. Tissuedistribution of paclitaxel after administration of liposome-encapsulatedpaclitaxel showed levels 10-fold higher in the spleen and 3.5-fold higher in theliver as compared to levels achieved with conventional paclitaxel. However, inkidneys, lungs, the brain, and lymph nodes, the paclitaxel concentrations weretwo to three times lower with liposome-encapsulated paclitaxel compared to thosewith conventional paclitaxel. The significant decrease in toxicity andincrease in plasma area under the curve and half-life with liposome-encapsulatedpaclitaxel indicate that this liposomal formulation may be a viable alternativeto the conventional preparation of paclitaxel for therapeutic use.
Paclitaxel Is First Liposomal Taxane to Enter Clinical Trials
Based on successful results obtained in preclinical studies ofliposome-encapsulated paclitaxel, a phase I clinical trial was conducted inpatients with advanced malignancies. Liposome-encapsulated paclitaxel is thefirst liposomal taxane to enter clinical trials.
Liposome-encapsulated paclitaxel was administered over 45minutes every 3 weeks without antiemetics. To date, 26 patients have beentreated at escalating dose levels: 3 patients at 90 mg/m2, 3 at 135mg/m2, 11 at175 mg/m2, 6 at 250 mg/m2, and 3 at 300 mg/m2.
A distinct toxicity profile was observed. Indeed, no clinicallysignificant neuropathies or myalgias have been observed at the MTD level (175mg/mg2). Alopecia was also absent at the MTD or less. Dose-limiting toxicitiesincluded mucositis at 300 mg/m2 (two patients); neutropenic sepsis, anaphylaxisat 250 mg/m2 (two patients); and anaphylaxis at 175 mg/m2 (one patient). Grade 4neutropenia and leukopenia and grade 3 thrombocytopenia and anemia were firstobserved at the 175 mg/m2 dose. Grade 3-4 mucositis and neutropenia wereobserved at liposome-encapsulated paclitaxel doses of 250 mg/m2 or higher.
Liposome infusion reactions included transient back pain andflushing in five patients and rigors in one patient, but since routinepremedication with diphenhydramine and hydrocortisone has been instituted, theseeffects have been mostly ameliorated. Two partial responses and three minorresponses have been observed.
Liposomes are safe and effective drug carriers. Liposomalencapsulation of cytotoxic drugs diminishes toxicity to normal tissue whilemaintaining therapeutic efficacy. The pharmacokinetic and pharmacodynamicprofiles of the free drug are favorably attenuated by liposomes. Throughimprovements and modifications of liposomal formulations, liposomes willcontinue to be important drug-delivery systems in oncology.
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