Inhibition of murine hepatic tumor growth by liposomes containing a lipophilic muramyl dipeptide

Different activities of antitumor immunomodulators to induce neutrophil adherence response

Functions of neutrophils, major participant in host defense mechanisms, are known to be regulated by various types of immunomodulators. Capacity of immunomodulators which are reported to show antitumor effect in vivo to induce neutorophil adherence response in vitro was investigated. Several bacterial immunomodulators (OK-432, Corynebacterium parvum, B.C.G.) and components of bacteria cell walls (lipopolysaccharide (LPS), lipid A, lipoteicoic acid, N-cell wall skelton (N-CWS), muramyl dipeptide (MDP)) and fungal polysaccharides (lentinan, zymosan A, etc.) were tested. Neutrophils prepared from peripheral blood of healthy men were incubated with each immunomodulator at 37°C for 60 min in 96 well plastic plates, then neutrophils adherent to substratum were stained by crystal violet and their optical density at 570 nm was measured as a parameter of neutrophil adherence. Although purified polysaccharides mainly prepared from fungi did not induce the adherent response, not only bacterial bodies and their components but also tumor necrosis factor-α (TNF-α) clearly induced it. On the base of these results, functional classification and typing of immunomodulators by different activities in neutrophil adherence was discussed.

Translation of peptidoglycan metabolites into immunotherapeutics

The discovery of defined peptidoglycan metabolites that activate host immunity and their specific receptors has revealed fundamental insights into host-microbe recognition and afforded new opportunities for therapeutic development against infection and cancer. In this review, we summarise the discovery of two key peptidoglycan metabolites, γ-d-glutamyl-meso-diaminopimelic acid (iE-DAP) and muramyl dipeptide and their respective receptors, Nod1 and Nod2, and review progress towards translating these findings into therapeutic agents. Notably, synthetic derivatives of peptidoglycan metabolites have already yielded approved drugs for chemotherapy-induced leukopenia and paediatric osteosarcoma; however, the broad effects of peptidoglycan metabolites on host immunity suggest additional translational opportunities for new therapeutics towards other cancers, microbial infections and inflammatory diseases.

Harnessing the untapped potential of nucleotide-binding oligomerization domain ligands for cancer immunotherapy

In the last decade, cancer immunotherapy has emerged as an effective alternative to traditional therapies such as chemotherapy and radiation. In contrast to the latter, cancer immunotherapy has the potential to distinguish between cancer and healthy cells, and thus to avoid severe and intolerable side‐effects, since the cancer cells are effectively eliminated by stimulated immune cells. The cytosolic nucleotide‐binding oligomerization domains 1 and 2 receptors (NOD1 and NOD2) are important components of the innate immune system and constitute interesting targets in terms of strengthening the immune response against cancer cells. Many NOD ligands have been synthesized, in particular NOD2 agonists that exhibit favorable immunostimulatory and anticancer activity. Among them, mifamurtide has already been approved in Europe by the European Medicine Agency for treating patients with osteosarcoma in combination with chemotherapy after complete surgical removal of the primary tumor. This review is focused on NOD receptors as promising targets in cancer immunotherapy as well as summarizing current knowledge of the various NOD ligands exhibiting antitumor and even antimetastatic activity in vitro and in vivo.

Tuftsin augments antitumor efficacy of liposomized etoposide against fibrosarcoma in Swiss albino mice

Anticancer drugs are generally plagued by toxic manifestations at doses necessary for control of various forms of cancer. Incorporating such drugs into liposomes not only reduces toxicity but also enhances the therapeutic index. Some antioxidants and potent immunomodulators have also been shown to impart significant antitumor activity presumably by nonspecific activation of the host immune system. In the present study, we evaluated augmentation of the antitumor activity of etoposide (ETP) by the immunomodulator tuftsin in Swiss albino mice with fibrosarcoma. The efficacies of the free form of ETP, liposomized ETP (Lip-ETP), and tuftsin-bearing liposomized ETP (Tuft-Lip-ETP) formulations were evaluated on the basis of tumor regression, effect on expression level of p53wt and p53mut, and survival of the treated animals. Tuft-Lip-ETP, when administered at a dosage of 10 mg/kg body weight/day for five days, significantly reduced tumor volume, delayed tumor growth, and also up-regulated the expression of p53wt. In contrast, although Lip-ETP delayed tumor growth, it did not decrease tumor size. The results of the present study suggest that tuftsin incorporation in drug-loaded liposomes is a promising treatment strategy for various forms of cancers, including fibrosarcoma.

Tuftsin augments antitumor efficacy of liposomized etoposide against fibrosarcoma in Swiss albino mice

Anticancer drugs are generally plagued by toxic manifestations at doses necessary for control of various forms of cancer. Incorporating such drugs into liposomes not only reduces toxicity but also enhances the therapeutic index. Some antioxidants and potent immunomodulators have also been shown to impart significant antitumor activity presumably by nonspecific activation of the host immune system. In the present study, we evaluated augmentation of the antitumor activity of etoposide (ETP) by the immunomodulator tuftsin in Swiss albino mice with fibrosarcoma. The efficacies of the free form of ETP, liposomized ETP (Lip-ETP), and tuftsin-bearing liposomized ETP (Tuft-Lip-ETP) formulations were evaluated on the basis of tumor regression, effect on expression level of p53wt and p53mut, and survival of the treated animals. Tuft-Lip-ETP, when administered at a dosage of 10 mg/kg body weight/day for five days, significantly reduced tumor volume, delayed tumor growth, and also up-regulated the expression of p53wt. In contrast, although Lip-ETP delayed tumor growth, it did not decrease tumor size. The results of the present study suggest that tuftsin incorporation in drug-loaded liposomes is a promising treatment strategy for various forms of cancers, including fibrosarcoma.

Liposomal phosphatidylserine inhibits tumor cytotoxicity of liver macrophages induced by muramyl dipeptide and lipopolysaccharide

Liposomes can very efficiently deliver immunomodulators to macrophages so as to induce tumor cytotoxicity. Liposomes most widely used for that purpose contain negatively charged lipids, in particular phosphatidylserine (PS), to enhance liposome uptake by the macrophages. We investigated the effect of three negatively charged liposomal lipids on the in vitro activation of liver macrophages to tumor cytotoxicity by muramyl dipeptide (MDP) and lipopolysaccharide (LPS). Both MDP- and LPS-induced tumor cytotoxicity towards murine colon adenocarcinoma cells were strongly inhibited by PS-containing liposomes. Under comparable conditions phosphatidylglycerol (DPPG)-containing or dicetyl phosphate (DCP)-containing liposomes did not inhibit or only marginally inhibited the induction of tumor cytotoxicity. We did not observe PS-mediated inhibition of tumor cell toxicity when the exposure of the macrophages to PS-liposomes was limited to the 4-h activation period prior to addition of the tumor target cells, suggesting that the inhibitory effect is accomplished at the level of the later stages of the activation process. Previously, we showed that macrophages which are activated to tumor cytotoxicity during a 24-h incubation with MDP become refractory to a second activation with MDP. Now we observed that simultaneous incubation with PS-containing liposomes partially prevents this refractoriness, which is also compatible with an interfering action of PS at a relatively late stage in the activation process. We conclude that PS, despite its reported stimulatory effect on liposome uptake by macrophages, can seriously antagonize the effectiveness of immunomodulating agents acting on macrophages. This bears relevance to the use of PS-containing liposomes as a vehicle for such agents. The results are discussed in perspective of earlier reported pharmacological effects of PS and its metabolites.

Histochemical and electron microscopic characterization of hepatic macrophage subfractions isolated from normal and liposomal muramyl dipeptide treated rats

Subfractions of the hepatic macrophage population, differing in cell size, were isolated from normal rats and rats treated with liposomal muramyl dipeptide (lipMDP) and analyzed histochemically and by ultrastructural peroxidase cytochemistry. The majority of cells in all subfractions of control rats displayed the ultrastructural endogenous peroxidase pattern of resident liver macrophages and showed positive staining with the general macrophage markers nonspecific esterase (NSE) and monoclonal antibody ED1. Heterogeneity in intensity of NSE and ED1 staining was observed among macrophages of different size. Generally, the intensity of NSE and ED1 staining decreased with decreasing cell size. After injection of lipMDP, we observed the appearance of a discrete subpopulation of cells in the liver in addition to the resident macrophages. These cells, containing a nucleus with a characteristic shape, were predominantly recovered in the small-sized fractions and were characterized by an immature ultrastructural macrophage morphology (no or only a few lysosomes and phagosomes) and a lack of ED1 reactivity, NSE, and endogenous peroxidase. We suggest an important role for these cells in lipMDP induced antitumor capacity of the liver.

Effect of membrane free fatty acid alterations on the adhesion of human colorectal carcinoma cells to liver macrophages and extracellular matrix proteins

Epidemiologic studies have linked diets high in animal fat with colon carcinogenesis. A number of animal tumor models have shown that diets rich in omega-3 fatty acids inhibit colon carcinogenesis while diets rich in omega-6 fatty acids promote tumor growth. This study examines whether modification of the membrane fatty acid composition of both moderately (CX-1) and poorly differentiated (MIP-101 and Clone A) human colorectal carcinoma cells alters their interaction with Kupffer cells and extracellular matrix proteins (collagen type IV, fibronectin and laminin). The cells were treated with 15-16 micrograms/ml of docosahexanoic acid (22:6, omega 3) or linoleic acid (18:2,omega 6). Gas chromatography showed significant alterations in the membrane fatty acid composition of the human colorectal cancer cell lines. Binding assays were performed by measuring adherence of 51Cr-labelled tumor cells to Kupffer cell monolayers or to immobilized proteins. Omega-3 treatment significantly decreased the Kupffer cell binding of only the CX-1 line while omega-6 treatment decreased binding of all three cell lines. In contrast both omega-3 and omega-6 treatment of MIP-101 cells decreased binding to the extracellular matrix proteins with the omega-6 effect being more pronounced. These results indicate that the binding characteristics of the colon cancer cells to both Kupffer cells and extracellular matrix proteins may be determined in part by the membrane fatty acid composition. Decreased adherence to extracellular matrix proteins may lead to increased cell motility and invasiveness. Since Kupffer cell binding precedes tumor cell phagocytosis and killing, decreased binding may improve tumor cell survival.

The effect of liver macrophages on in vitro cytolytic activity of 5FU and FUdR on colon carcinoma cells: evidence of macrophage activation

While investigating the effects of 5-fluorouracil (5FU) and 5-fluoro-2'-deoxyuridine (FUdR) on the tumoricidal state of rat liver macrophages activated in vitro by means of liposome-encapsulated muramyl dipeptide (MDP), we observed that 5FU in combination with macrophages produced substantially higher extents of cytolytic activity on tumor cells than 5FU alone. In contrast, FUdR failed to produce this effect; rather, at relatively low FUdR concentrations, lytic activity in the presence of macrophages was even significantly diminished as compared with FUdR in the absence of macrophages. Both 5FU and FUdR were able to enhance the cytolytic activity of macrophages activated by liposome-encapsulated MDP. This finding indicates that, rather than inhibiting the activation of macrophages by liposomal MDP, 5FU can act as a stimulator of macrophage activation by itself. This is further supported by the observations that (i) in combination with 5FU, the secretion of TNF induced by liposomal MDP was synergistically enhanced and (ii) that a second treatment of macrophages with the drug, 24 h after the first, fails to produce increased macrophage cytotoxicity. Our results also show that neither 5FU nor FUdR are likely to unfavorably influence the induction of cytotoxic activity of the macrophages. Rather, combinations of 5FU or FUdR and liposomal MDP may result in an additive or synergistic tumoricidal effect.

Therapy of disseminated melanoma by liposome-activated macrophages

The uncontrolled growth of metastases is a major cause of death from melanoma. Metastases arise from the nonrandom spread of specialized malignant cells that preexist within a primary neoplasm. Macrophages are activated to become tumoricidal by interaction with phospholipid vesicles (liposomes) containing immunomodulators recognize and destroy neoplastic cells in vitro and in vivo while leaving nonneoplastic cells unharmed. The mechanism(s) by which macrophages discriminate between tumorigenic and normal cells is independent of tumor cell characteristics such as immunogenicity, metastatic potential, and sensitivity to cytotoxic drugs. Moreover, macrophage destruction of tumor cells apparently is not associated with the development of tumor cell resistance. Intravenously administered liposomes are cleared from the circulation by phagocytic cells. The administration of such liposomes has been shown to activate macrophages in situ and to bring about eradication of cancer metastases in several experimental tumor systems and in dogs with spontaneous osteogenic sarcoma. Macrophage destruction of metastases in vivo is significant, provided that the tumor burden at the start of treatment is minimal. For this reason, we have been investigating various methods to achieve maximal cytoreduction in metastases by modalities such as chemotherapy or radiotherapy prior to systemic activation of macrophages. Macrophage-directed therapy has been studied in several human protocols, yielding important biological information about the safe use of liposome-encapsulated macrophage activators in cancer patients.

Anti-metastatic activity in vivo of MDP-L-alanyl-cholesterol (MTP-Chol) entrapped in nanocapsules

A lipophilic muramylpeptide (MTP-Chol), capable of rendering macrophages cytostatic towards tumour cells, was encapsulated within polyisobutylcyanoacrylate nanocapsules and administered to mice carrying an experimental model of liver metastasis. Treatment by intravenous injections twice a week beginning before the establishment of metastases significantly reduced the number of liver colonies. Treatment started later was less effective. The dose of MTP-Chol in each injection, and the tumour burden in the mice did not change the percentage inhibition of metastases significantly. Anti-metastatic activity was also observed after administering nanocapsules containing MTP-Chol by the oral route.

Liposomal muramyl dipeptide therapy of experimental M5076 liver metastases in mice

The effectiveness of N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) or of liposomes containing a lipophilic MDP derivative, MDP-glyceroyldipalmitate MDP-GDP in inhibiting the growth of M5076 reticulum cell sarcoma liver metastases in C57BL/6 mice has been determined. MDP (100 micrograms) or liposomal MDP-GDP (2.5 mumol containing 1 microgram) were equally effective in inhibiting liver metastatic growth when given as a single treatment 3 days before tumor cell injection. Therapeutic treatment, initiated 3 days after tumor cell injection and continued for a period of 2 weeks, failed to inhibit metastatic growth. Activation of thioglycollate-elicited peritoneal macrophages or Kupffer cells in vitro with MDP or liposomal MDP-GDP resulted in the expression of tumoricidal activity against M5076 tumor cells. Adoptive cellular therapy with four injections of 2 x 10(6) macrophages was ineffective: activation of the macrophages with either MDP or liposomal MDP-GDP prior to injection was effective in inhibiting liver metastatic growth. Incorporation of the macrophage toxin dichlorodimethylene diphosphonate within liposomes containing MDP-GDP abolished the ability of such liposomes to induce macrophage or Kupffer cell tumoricidal activity in vitro as well as the antitumor activity when administered 3 days before tumor cell challenge.

In situ activation of mouse macrophages and therapy of spontaneous renal cell cancer metastasis by liposomes containing the lipopeptide CGP 31362

We determined whether the intravenous administration of multilamellar vesicle liposomes (MLV) containing a lipopeptide analogue of a fragment from the cell wall of gram-negative bacteria (CGP 31362) can render BALB/c mouse alveolar macrophages tumoricidal in situ and reduce the incidence of spontaneous lung metastasis of syngeneic renal carcinoma (RENCA) cells. Alveolar macrophages (a) incubated in vitro with MLV containing CGP 31362 (MLV-31362) and (b) harvested from mice injected i.v. with MLV-31362 were rendered cytotoxic against the RENCA cells. Maximum cytotoxic activity of the macrophages was induced by injecting 5 mumol MLV consisting of 250 mg phospholipids and 0.5 mg CGP 31362. The single i.v. injection of 5 mumol MLV-31362 produced activation of macrophages that lasted for up to 4 days. Repeated i.v. injections of MLV-31362 produced a continuous antitumor activity in alveolar macrophages. To study the lipopeptide's effects on metastasis, we injected the left kidneys of BALB/c mice with RENCA cells. The kidney with growing tumor was resected 10 days later and, after a further 2 days, groups of mice were injected i.v. with MLV-31362 or with MLV-HBSS (twice weekly for 3 weeks). Treatment with MLV-31362 significantly decreased the median number of spontaneous lung metastases. These data demonstrate that the systemic administration of MLV-31362 can activate murine lung macrophages in situ and reduce the incidence of spontaneous RENCA lung metastases.

Comparative efficacy of liposomes containing synthetic bacterial cell wall analogues for tumoricidal activation of monocytes and macrophages

We examined the activation to the tumoricidal state of normal mouse peritoneal exudate macrophages, bone marrow macrophages, and human blood monocytes by liposomes containing either lipophilic muramyl tripeptide (CGP 19,835) or a new synthetic analogue of lipoprotein from gram-negative bacteria outer wall, CGP 31,362, or combinations of the two. The superiority of liposomes containing the synthetic lipopeptide over liposomes containing lipophilic muramyl tripeptide for in vitro activation of monocytes and macrophages was demonstrated in several experiments. First, liposome-CGP-19,835 activated monocytes only in the presence of interferon-gamma, whereas activation with liposome-CGP 31,362 was interferon-independent. Second, activation of both mouse macrophages and human blood monocytes by liposome-CGP 31,362 occurred at a lower liposomal concentration than that by liposome-CGP 19,835. Third, monocytes incubated with liposome-CGP 31,362 released both tumor necrosis factor (TNF) and interleukin-1 activities, whereas monocytes treated with liposome-CGP 19,835 (in the absence of interferon-gamma) released only TNF activity. These data suggest that liposomes containing the synthetic lipopeptide CGP 31,362 are superior to liposomes containing CGP 19,835 for systemic activation of macrophages.

Macrophages and cancer

The uncontrolled growth of metastases resistant to conventional therapeutic modalities is a major cause of death from cancer. Data from our laboratory and others indicate that metastases arise from the nonrandom spread of specialized malignant cells that preexist within a primary neoplasm. These metastases can be clonal in their origin, and different metastases can originate from different progenitor cells. In addition, metastatic cells can exhibit an increased rate of spontaneous mutation compared with benign nonmetastatic cells. These data provide an explanation for the clinical observation that multiple metastases can exhibit different sensitivities to the same therapeutic modalities. These findings suggest that the successful therapy of disseminated metastases will have to circumvent the problems of neoplastic heterogeneity and the development of resistance. Appropriately activated macrophages can fulfill these demanding criteria. Macrophages can be activated to become tumoricidal by interaction with phospholipid vesicles (liposomes) containing immunomodulators. Tumoricidal macrophages can recognize and destroy neoplastic cells in vitro and in vivo, leaving nonneoplastic cells uninjured. Although the exact mechanism(s) by which macrophages discriminate between tumorigenic and normal cells is unknown, it is independent of tumor cell characteristics such as immunogenicity, metastatic potential, and sensitivity to cytotoxic drugs. Moreover, macrophage destruction of tumor cells apparently is not associated with the development of tumor cell resistance. Macrophages are found in association with malignant tumors in a definable pattern, suggesting that the most direct way to achieve macrophage-mediated tumor regression is in situ macrophage activation. Intravenously administered liposomes are cleared from the circulation by phagocytic cells, including macrophages, so when liposomes containing immunomodulators are endocytosed, cytotoxic macrophages are generated in situ. The administration of such liposomes in certain protocols has been shown to bring about eradication of cancer metastases. Macrophage destruction of metastases in vivo is significant, provided that the total tumor burden at the start of treatment is minimal. For this reason, we have been investigating various methods to achieve maximal cytoreduction in metastases by modalities such as chemotherapy or radiotherapy prior to macrophage-directed therapy. It is important to note that even the destruction of 99.9% of cells in a metastasis measuring 1 cm2 would leave 10(6) cells to proliferate and kill the host. The ability of tumoricidal macrophages to distinguish neoplastic from bystander nonneoplastic cells presents an attractive possibility for treatment of the few tumor cells which escape destruction by conventional treatments. Macrophage-directed therapy has been studied in several human protocols, yielding important biological information about the use of liposome-encapsulated macrophage activators in cancer patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Kupffer cells and liver metastasis. Optimization and limitation of activation of tumoricidal activity

Kupffer cells, tissue-fixed macrophages located in the sinusoids of the liver, represent the highest concentration of mononuclear phagocytes in the body. Their ability to act as scavengers of particulate material in the blood has given rise to speculation that they play a role in controlling hepatic metastases derived from blood-borne tumor cells. Circumstantial evidence for such a role has been obtained from animal studies where Kupffer cell function has been compromised or inhibited, and from anecdotal clinical observations. Current evidence suggests that Kupffer cells are capable of nonspecifically eliminating some circulating tumor cells from the circulation via phagocytosis. This surveillance mechanism would appear to be limited in capacity, and subject to a number of external factors. Recent studies have demonstrated that Kupffer cells can be activated to a tumoricidal state via the administration of biological response modifiers such as gamma interferon or muramyl peptides. The localization of liposomes within Kupffer cells after systemic administration has provided a considerable stimulus for the efficient targeting of macrophage-activating compounds to these cells. Such therapeutic intervention, while capable of inducing Kupffer cell tumoricidal activity in situ and inhibiting tumor growth, is limited with respect to the location of the tumor cells (sinusoidal versus parenchymal) and to the size of the metastatic nodule. Therapeutic intervention using liposomes containing macrophage-activating agents may only be of benefit in patients with minimal tumor load who are at risk for hepatic metastases, rather than those patients who already have clinically detectable liver tumors.