Future Prospects for Stealth Liposomes in Cancer Therapy
Future Prospects for Stealth Liposomes in Cancer Therapy
Doxorubicin (Adriamycin) is among the most active single agents used in the treatment of advanced breast cancer and other solid malignancies. However, the therapeutic index of this drug is often limited by its toxicities. These include nausea and vomiting and subacute toxicities such as alopecia, mucositis, and bone marrow suppression. Neutropenia is frequently encountered with doxorubicin at the dose intensities needed to achieve a meaningful response rate. Severe neutropenia can lead to febrile episodes and occasionally to septic infections that can be life-threatening. Moreover, the potential for irreversible cardiotoxicity limits cumulative exposure of doxorubicin to doses of less than 450 to 550 mg/m2.
Treatment strategies for advanced cancers vary geographically and by treatment centers. Depending on the disease setting, the goal of systemic chemotherapy can range from palliation to an intent to cure. Many medical oncologists believe that improved survival is not a realistic objective for systemic chemotherapy in patients who present with a high disease burden and/or multiple visceral metastatic sites. Clinical data support the antitumor activity of agents such as doxorubicin. However, some argue that the benefit of modestly improved survival among a small number of patients does not outweigh the risk of toxicity to all patients who receive such therapy, particularly elderly and/or frail patients who are less likely to tolerate therapy.
In some treatment centers, therefore, patients with advanced cancers are not treated with chemotherapy, but are given supportive care to palliate the signs and symptoms of the disease. This debate has raised awareness of the importance of patients quality of life and has driven a search for treatment options that provide responses in a significant number of patients, and/or delay disease progression for a meaningful period of time with minimal toxicity.
Preclinical studies of pegylated liposomal doxorubicin (PEG-LD) (Doxil) and experience in the treatment of Kaposis sarcoma (KS) suggest that compared with standard doxorubicin, pegylated liposomes (Stealth) deliver a greater proportion of an injected dose of doxorubicin to tumor sites. If this were also the case in other human cancers, one might reasonably expect PEG-LD to have similar antitumor activity to that of doxorubicin, but at a lower dose intensity, which would produce less severe toxicity.
This expectation provides a rationale, for example, for developing PEG-LD as single-agent therapy for advanced breast cancer among elderly patients. The goal would be a tumor response rate comparable with doxorubicin but with a dose and schedule of PEG-LD that minimizes the frequency and severity of nausea, vomiting, neutropenia, mucositis, alopecia, and cardiotoxicity.
This reasoning may extend to combinations of PEG-LD with several noncytotoxic biologic agents, which are the subject of ongoing clinical investigations. In general, such agents produce minimal toxicity but are not sufficiently active on their own.
Humanized monoclonal antibodies directed against receptors overexpressed on malignant cells such as p185HER2 or the EGFr, appear to fall into this category.[2,3] Peptide and protein inhibitors of angiogenesis may also need to be combined with a cytotoxic agent for meaningful antitumor activity. By combining these agents with PEG-LD, the good tolerance to each may be maintained while additive or perhaps synergistic antitumor activity may be achieved.
This theory has been tested preclinically using a combination of the anti-EGFr monoclonal antibody C225 and PEG-LD against a xenograft of the A431 tumor implanted in nude mice. As shown in Figure 1, a combination of PEG-LD and the C225 antibody produced an antitumor effect superior to either of these agents alone, to doxorubicin alone, and to a combination of the antibody plus doxorubicin. These results support the therapeutic rationale of combining PEG-LD with biologics.
Two principles guide the selection of cytotoxic agents for use in combination regimens: (1) they should be individually active in the selected tumor type and (2) have nonoverlapping dose-limiting toxicities. PEG-LD has been shown to be active as a single agent in refractory ovarian cancer and as primary therapy for recurrent breast cancer.[1,5] While generally manageable in both of these settings, epithelial cell toxicity manifesting itself as palmar-plantar erythrodysesthesia (PPE, hand-foot syndrome) may limit the amount of PEG-LD patients are able to tolerate. Therefore, it is reasonable to explore combinations of PEG-LD with other agents proven to be active in these tumor types but with toxicities other than skin.
PEG-LD Plus Vinorelbine
The dose-limiting toxicity of vinorelbine tartrate (Navelbine) is granulocytopenia. In combination with doxorubicin, vinorelbine produced a 57% overall objective response rate as first-line therapy for advanced breast cancer. However, the incidence of grade 4 granulocytopenia was 83%, with 8% requiring hospitalization due to febrile neutropenia and one septic death. Substitution of doxorubicin with PEG-LD in this combination is being explored as a means of maintaining the favorable tumor response of the combination while reducing the incidence of hematological toxicity. Vinorelbine would not be expected to contribute to the skin toxicity seen with PEG-LD.
PEG-LD Plus Gemcitabine
Gemcitabine (Gemzar) given alone has been reported to have meaningful activity in non-small-cell lung, breast, ovarian, and head-and-neck cancers and a favorable safety profile. Only modest levels of traditional cytotoxicities such as myelosuppression, nausea and vomiting, and alopecia are seen with gemcitabine administered on a weekly schedule at doses less than 1,000 mg/m2. Although maculopapular skin rashes have been reported secondary to injection of gemcitabine, particularly in Asian patients, the timing of the appearance of the rash (within 1 to 2 days of injection) and the anatomical sites (chest, abdominal wall, upper arms and thighs) suggest that the underlying cause of this toxicity will not aggravate the palmar-plantar erythrodysesthesia seen with PEG-LD. Given the non-overlapping toxicity profiles of gemcitabine and PEG-LD and the complimentary single-agent activity of the two in breast and ovarian cancer, combinations of these agents are being tested in ongoing pilot phase I dose-finding trials.
PEG-LD Plus Paclitaxel
The excitement generated by Gianni et al who reported a greater than 90% objective response rate in metastatic breast cancer for a paclitaxel/doxorubicin combination is tempered by the rather unfavorable side effect profile of this regimen. Severe, febrile neutropenia was common and peripheral neuropathy occurred in one third of the patients. Perhaps more troubling was the development of congestive heart failure (CHF), albeit reversible in 18% of women after a median of 480 mg/m2 doxorubicin. These results raise the specter of paclitaxel-related enhancement of doxorubicin cardiotoxicity.
Studies of single-agent doxorubicin have demonstrated that CHF is very uncommon at cumulative doses of less than 500 mg/m2. Dosing schedules of doxorubicin designed to lower peak plasma concentration are known to reduce cardiotoxicity, suggesting that the peak dose (Cmax) in plasma after a typical doxorubicin infusion contributes to development of this toxicity. Peak levels of bioavailable doxorubicin are substantially reduced following PEG-LD administration, and preclinical results indicate that PEG-LD causes fewer morphological changes in heart tissues relative to free doxorubicin in three species: rat, dog, and rabbit.
These encouraging preclinical findings are supported by results of a pilot clinical study by Berry et al. These authors performed endomyocardial biopsies on a series of AIDS-KS patients who had received cumulative doses of PEG-LD ranging from 469 to 860 mg/m2. This group was compared with two historical control groups treated with doxorubicin. One group was matched with respect to cumulative exposure to doxorubicin and the other with respect to both cumulative dose and administered dose. Biopsy specimens were examined microscopically and scored according to the 0 (no evidence of anthracycline-specific damage) to 3.0 (severe diffuse myocyte damage; greater than 35% of all cells) scale introduced by Billingham and colleagues. The mean biopsy score for the PEG-LD group was 0.5 (± 0.6). For the two control groups the mean biopsy scores were 2.1 (± 0.7) and 1.4 (± 0.65), respectively. The comparisons between the mean PEG-LD scores and the control groups were highly significant in both cases (P < .001).
These finding support the rationale for combining paclitaxel with PEG-LD. Both drugs have demonstrated activity in breast and ovarian cancer. With respect to toxicities, the incidence of severe neutropenia and peripheral neuropathy are lower for PEG-LD than for paclitaxel. Also paclitaxel causes relatively little skin toxicity. In addition, preclinical and early clinical biopsy results strongly suggest that PEG-LD produces less damage to the myocardium relative to comparable cumulative doses of doxorubicin. Thus the cardioprotective effect of PEG-LD may translate into a reduced risk of cardiotoxicity relative to the highly active paclitaxel-doxorubicin combination. Based on these considerations, several phase I dose-finding trials of PEG-LD and paclitaxel have been launched.