Effect of liposomally trapped antitumour drugs on a drug-resistant mouse lymphoma in vivo

Clinical potential of elacytarabine in patients with acute myeloid leukemia

Acute myeloid leukemia (AML) has been treated for over four decades with standard induction chemotherapy including seven days of cytosine arabinoside (cytarabine, ara-C) infusion. Cytarabine, while effective in killing leukemic cells, is subject to development of several resistance mechanisms rendering the drug ineffective in many patients. Elacytarabine, a lipophilic 5'-elaidic acid ester or nucleoside analogue of cytosine arabinoside, was created with the intent of overcoming resistance mechanisms including reduced expression of the human equilibrative nucleoside transporter 1 (hENT1) required for cytarabine entry into cells, as well as increased activity of cytidine deaminase (CDA) which breaks down the active metabolite of cytarabine, ara-CTP. Elacytarabine enters cells independently of transporters, has a longer half life compared with cytarabine and is not subject to deactivation by CDA. Preclinical data were encouraging although subsequent clinical studies have failed to show superiority of elacytarabine compared with standard of care as monotherapy in patients with AML. Clinical trials utilizing elacytarabine in combination with anthracyclines are ongoing. Use of hENT1 expression as a predictive marker for cytarabine or elacytarabine response has been studied with no conclusive validation to date. Despite promising early results, the jury is still out in regards to this novel agent as an effective alternative to standard cytarabine therapy in acute leukemias, especially in combination with additional agents such as anthracyclines.

Cholesterol as a bilayer anchor for PEGylation and targeting ligand in folate-receptor-targeted liposomes

Phospholipids have been extensively evaluated as an anchor for both PEGylation and receptor-targeting in liposomal formulations. However, cholesterol, another important component in biomembranes, has not been fully investigated as an alternative anchor. In this study, the potential role of cholesterol for anchoring PEG and folate was investigated. Cholesterol derivatives were synthesized for PEGylation (mPEG-cholesterol) and folate receptor (FR) targeting (folate-PEG-cholesterol) and incorporated into the bilayer of FR-targeted liposomal doxorubicin. The colloidal stability of these cholesterol derivative-containing liposomes was superior to non-PEGylated liposomes, indicating that steric barrier provided by mPEG-cholesterol can efficiently inhibit aggregation of liposomes. FR-targeting activity of these liposomes was demonstrated by in vitro cell-binding studies on FR-overexpressing KB cells. In addition, in vivo circulation of cholesterol-anchored liposomes was prolonged compared to non-PEGylated liposomes. These studies suggest that cholesterol is a viable bilayer anchor for synthesis of PEGylated and FR-targeted liposomes.

Cancer chemotherapy administered by activated carbon particles and liposomes

In cancer chemotherapy, a specific drug delivery system is expected since many anticancer drugs show toxicity against not only cancer cells but also against normal tissues. The dosage form comprising anticancer drugs adsorbed on activated carbon particles or encapsulated in liposomes is developed as a drug-delivery system which enhances the therapeutic efficacy and reduces the adverse effects. The dosage forms are versatile in size and electric charge, so that large amounts of the drugs are distributed to the "targeted" organs or tissues and lesser amounts are distributed to the whole body. The dosage forms are designed to release the drugs slowly for a long time at local sites. Through this process, practical use of the dosage forms as an anticancer drug carrier results in an enhancement of anticancer efficacy on the local lesion and a decrease of systemic toxicity.

Liposomes as drug carrier system for cis-diamminedichloroplatinum (II). II. Antitumor activity in vivo, induction of drug resistance, nephrotoxicity and Pt distribution

In this study we have investigated the use of liposomes as a drug carrier system for cis-diamminedichloroplatinum(II) (cis-DDP) in order to reduce the nephrotoxicity with preservation of antitumor activity. Liposomes (PC/PS/Chol 10:1:4) were prepared using hydration media containing no or a relatively low concentration of NaCl. It was found that cis-DDP containing liposomes (lip cis-DDP) injected i.v. to IgM immunocytoma-bearing LOU/M rats at a dose of 1 mg cis-DDP/kg (cumulative dose 7 mg cis-DDP/kg) showed less antitumor activity than the free drug. The optimal cumulative dose of free cis-DDP for induction of antitumor activity in this tumor system is 7 mg/kg (7 X 1 mg/kg). At a dose of 2 mg lip cis-DDP/kg (cumulative dose 14 mg cis-DDP/kg) the antitumor activity could not be increased by choosing another phospholipid composition of the liposomes [DPPC/DPPG/Chol (10:1:10)]. cis-DDP incorporated in DPPC/DPPG/Chol liposomes showed a similar antitumor activity to cis-DDP incorporated in PC/PS/Chol liposomes. After an i.v. dose of 2 mg lip cis-DDP/kg (PC/PS/Chol) kidney damage was less compared to the treatment with free cis-DDP (1 mg/kg). However, after a single dose of 2 mg cis-DDP/kg or a cumulative dose of 8 or 16 mg cis-DDP/kg, kidneys of rats treated with lip cis-DDP contained twice as much Pt as after treatment with free cis-DDP. Moreover, after treatment with lip cis-DDP, a twofold increase of the amount of Pt in tumor tissue was measured. In vitro studies with Pt recovered from spleens obtained from rats treated with lip cis-DDP i.v. showed that based on the equal amounts of Pt recovered the antitumor activity of the recovered Pt was reduced, indicating inactivation of cis-DDP in vivo. As during treatment with free cis-DDP, recurrence of the tumor was observed during the continued treatment with lip cis-DDP. It was found that these recurrent tumors were resistant to further therapy with cis-DDP. In conclusion, cis-DDP encapsulation into liposomes decreased the nephrotoxicity. The antitumor activity of cis-DDP is preserved by liposome encapsulation when it was used at a dose of 2 mg/kg, but it was reduced in terms of earlier onset of regrowth.

Treatment of murine L1210 lymphoid leukemia and melanoma B16 with lipophilic cytosine arabinoside prodrugs incorporated into unilamellar liposomes

Lipophilic prodrugs of I-beta-D-arabinofuranosyl cytosine (Ara-C), namely N4- and 5'-oleyl-I-beta-D-arabinofuranosyl cytosine (N4-oleyl-ara-C, 5'-oleyl-ara-C) and N4-palmitoyl-I-beta-D-arabinofuranosyl cytosine (N4-palm-ara-C) were incorporated into liposomes of various lipid compositions. The phospholipid vesicles were prepared by controlled dialysis of lipid/prodrug/detergent micelles yielding homogeneous and stable unilamellar liposomes. The liposome size ranged from 70 to 120 nm depending on the lipid composition and the amounts of prodrug incorporated. Depending on the total amount of micellized Ara-C prodrugs, a maximal incorporation rate of 250 micrograms prodrug per mg egg phosphatidylcholine was achieved. The incorporation efficiency was in the range of 85 to 97%. The in vivo antitumor activity of the liposome preparations against L1210 lymphoid leukemia was clearly superior by factors of 2-8 depending on the therapy schedule and route of drug application. The treatment of melanoma B16 with free Ara-C as well as with the prodrug-liposomes exhibit rather weak antitumor activity. Drug application by means of prodrug-liposomes yields equal or higher tumor-inhibitory effects at drug concentrations which were 2-4 times lower than those of free Ara-C. Although drug tolerance and myelosuppression studies with Swiss albino mice revealed that the prodrug-liposomes were 5-10 times more toxic than free Ara-C, a substantial improvement of efficiency could be observed. The incorporation of the ara-C prodrugs into liposomes provides protection against fast degradation and systemic elimination which might be an explanation for the improved antitumor effect of these preparations.

Superior therapeutic activity of liposome-associated adriamycin in a murine metastatic tumour model

We have examined the anti-tumour activity of liposome-entrapped Adriamycin in a murine metastatic tumour model produced by i.v. inoculation of J-6456 lymphoma cells and affecting predominantly the liver. Sonicated liposomes containing phosphatidylcholine, a negatively-charged phospholipid and cholesterol were used in these experiments. Liposome-entrapped Adriamycin was more effective than free Adriamycin at equivalent doses of the drug. The superior therapeutic effect of the liposome-associated drug was manifest, either with a single i.v. treatment using a dose bordering the toxicity threshold of free Adriamycin or with a multi-injection schedule using smaller doses. Based on the growth kinetics data of the J-6456 lymphoma, our results indicate that tumour cell killing was enhanced by a factor of approximately 100 using the liposome associated form of Adriamycin. Histopathologic studies in mice bearing well-established metastases of the J-6456 lymphoma in liver and spleen indicated that the extent and duration of pathologic remission were significantly improved in mice receiving the liposome-entrapped drug as compared to mice receiving free drug. No significant differences in the anti-tumour effect of liposome entrapped Adriamycin were observed replacing phosphatidylserine by phosphatidylglycerol and reducing the cholesterol:phospholipid molar ratio from 100% to 25%. In contrast to the metastatic tumour model, liposome-entrapped Adriamycin was significantly less effective than free Adriamycin on the local i.m. growth of the J-6456 tumour. Altogether the survival and histopathological data presented suggest that, with regard to a group of neoplastic conditions with a predominant pattern of liver dissemination, a substantial increase in the therapeutic index of Adriamycin can be achieved in a selective manner with the use of liposomes.