Pharmacokinetics and metabolism of muramyl dipeptide and nor-muramyl dipeptide [3H-labelled] in the mouse

Highlights into historical and current immune interventions for cancer

Immunotherapy is an additional pillar when combined with traditional standards of care such as chemotherapy, radiotherapy, and surgery for cancer patients. It has revolutionized cancer treatment and rejuvenated the field of tumor immunology. Several types of immunotherapies, including adoptive cellular therapy (ACT) and checkpoint inhibitors (CPIs), can induce durable clinical responses. However, their efficacies vary, and only subsets of cancer patients benefit from their use. In this review, we address three goals: to provide insight into the history of these approaches, broaden our understanding of immune interventions, and discuss current and future approaches. We highlight how cancer immunotherapy has evolved and discuss how personalization of immune intervention may address present limitations. Cancer immunotherapy is considered a recent medical achievement and in 2013 was selected as the "Breakthrough of the Year" by Science. While the breadth of immunotherapeutics has been rapidly expanding, to include the use of chimeric antigen receptor (CAR) T-cell therapy and immune checkpoint inhibitor (ICI) therapy, immunotherapy dates back over 3000 years. The expansive history of immunotherapy, and related observations, have resulted in several approved immune therapeutics beyond the recent emphasis on CAR-T and ICI therapies. In addition to other classical forms of immune intervention, including human papillomavirus (HPV), hepatitis B, and the Mycobacterium bovis Bacillus Calmette-Guérin (BCG) tuberculosis vaccines, immunotherapies have had a broad and durable impact on cancer therapy and prevention. One classic example of immunotherapy was identified in 1976 with the use of intravesical administration of BCG in patients with bladder cancer; resulting in a 70 % eradication rate and is now standard of care. However, a greater impact from the use of immunotherapy is documented by the prevention of HPV infections that are responsible for 98 % of cervical cancer cases. In 2020, the World Health Organization (WHO) estimated that 341,831 women died from cervical cancer [1]. However, administration of a single dose of a bivalent HPV vaccine was shown to be 97.5 % effective in preventing HPV infections. These vaccines not only prevent cervical squamous cell carcinoma and adenocarcinoma, but also oropharyngeal, anal, vulvar, vaginal, and penile squamous cell carcinomas. The breadth, response and durability of these vaccines can be contrasted with CAR-T-cell therapies, which have significant barriers to their widespread use including logistics, manufacturing limitations, toxicity concerns, financial burden and lasting remissions observed in only 30 to 40 % of responding patients. Another, recent immunotherapy focus are ICIs. ICIs are a class of antibodies that can increase the immune responses against cancer cells in patients. However, ICIs are only effective against tumors with a high mutational burden and are associated with a broad spectrum of toxicities requiring interruption of administration and/or administration corticosteroids; both of which limit immune therapy. In summary, immune therapeutics have a broad impact worldwide, utilizing numerous mechanisms of action and when considered in their totality are more effective against a broader range of tumors than initially considered. These new cancer interventions have tremendous potential notability when multiple mechanisms of immune intervention are combined as well as with standard of care modalities.

Uptake, recognition and responses to peptidoglycan in the mammalian host

Microbiota, and the plethora of signalling molecules that they generate, are a major driving force that underlies a striking range of inter-individual physioanatomic and behavioural consequences for the host organism. Among the bacterial effectors, one finds peptidoglycan, the major constituent of the bacterial cell surface. In the steady-state, fragments of peptidoglycan are constitutively liberated from bacterial members of the gut microbiota, cross the gut epithelial barrier and enter the host system. The fate of these peptidoglycan fragments, and the outcome for the host, depends on the molecular nature of the peptidoglycan, as well the cellular profile of the recipient tissue, mechanism of cell entry, the expression of specific processing and recognition mechanisms by the cell, and the local immune context. At the target level, physiological processes modulated by peptidoglycan are extremely diverse, ranging from immune activation to small molecule metabolism, autophagy and apoptosis. In this review, we bring together a fragmented body of literature on the kinetics and dynamics of peptidoglycan interactions with the mammalian host, explaining how peptidoglycan functions as a signalling molecule in the host under physiological conditions, how it disseminates within the host, and the cellular responses to peptidoglycan.

A muramidase from Acremonium alcalophilum hydrolyse peptidoglycan found in the gastrointestinal tract of broiler chickens

This study evaluates peptidoglycan hydrolysis by a microbial muramidase from the fungus Acremonium alcalophilum in vitro and in the gastrointestinal tract of broiler chickens. Peptidoglycan used for in vitro studies was derived from 5 gram-positive chicken gut isolate type strains. In vitro peptidoglycan hydrolysis was studied by three approaches: (a) helium ion microscopy to identify visual phenotypes of hydrolysis, (b) reducing end assay to quantify solubilization of peptidoglycan fragments, and (c) mass spectroscopy to estimate relative abundances of soluble substrates and reaction products. Visual effects of peptidoglycan hydrolysis could be observed by helium ion microscopy and the increase in abundance of soluble peptidoglycan due to hydrolysis was quantified by a reducing end assay. Mass spectroscopy confirmed the release of hydrolysis products and identified muropeptides from the five different peptidoglycan sources. Peptidoglycan hydrolysis in chicken crop, jejunum, and caecum samples was measured by quantifying the total and soluble muramic acid content. A significant increase in the proportion of the soluble muramic acid was observed in all three segments upon inclusion of the microbial muramidase in the diet.

Pharmacokinetics and feeding responses to muramyl dipeptide in rats

N-acetyl-muramyl-L-alanine-D-isoglutamine or muramyl dipeptide (MDP) is the minimally active subunit of bacterial peptidoglycan. During a systemic infection, the involvement of MDP has been demonstrated in food intake depression by the macrophage hydrolysis of Gram-positive bacteria. Under normal conditions, mammals are constantly exposed to the release of endogenous MDP from degraded gut flora and that of exogenous MDP from the diet. However, MDP digestion and absorption in the gastrointestinal tract are not fully understood, and their physiological significance needs to be clarified. After gavage (1.5 mg/kg), very low levels of MDP were found in the systemic circulation of rats and feeding patterns were not altered. In contrast, after the intraperitoneal injection of a similar dose, a depression in food intake was observed. The rats reduced their meal frequency and constant feeding rate, showing signs of satiety. The behavioral satiety sequence (BSS) was modified by behavioral changes, similar to those which appear during sickness, such as an increase in resting and a reduction in grooming. Our data suggest that the hypophagic effect of MDP may result from satiety and sickness behavior.

Adjuvant activity of peptidoglycan monomer and its metabolic products

Peptidoglycan monomer (PGM) is a natural compound of bacterial origin. It is a non-toxic, non-pyrogenic, water-soluble immunostimulator potentiating humoral immune response to ovalbumin (OVA) in mice. It is fast degraded and its metabolic products-the pentapeptide (PP) and the disaccharide (DS)-are excreted from the mammalian organism upon parenteral administration. The present study investigates: (a). whether PGM could influence the long-living memory generation; (b). whether metabolic products retain adjuvant properties of the parent compound and contribute to its adjuvanticity. We report now that mice immunised twice with OVA+PGM had significantly higher anti-OVA IgG levels upon challenge with antigen alone 6 months later in comparison to control group immunised with OVA only. PP and DS were prepared enzymatically in vitro as apyrogenic and chemically pure compounds. When mice were immunised with OVA plus PP and DS, respectively, the level of anti-OVA IgGs in sera was not higher than in mice immunised with OVA alone, while PGM raised the level of specific antibodies. Results implicate that the adjuvant active molecule, capable of enhancing long-living memory generation, is PGM itself, and none of its metabolic products.

Influence of adjuvant-active peptidoglycan monomer on specific T cell responses in mice

Peptidoglycan monomer (PGM) originating from Brevibacterium divaricatum is a non-toxic, non-pyrogenic, water-soluble immunostimulator. It potentiates humoral immune response to ovalbumin (OVA) in mice upregulating both immunoglobulin (IgG) 1 and IgG2a antibody subclasses. This study concerns the influence of PGM on T cell activation and cytokine networks in response to OVA. OVA-specific proliferative response as well as interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) secretion in lymph node cell cultures of immunised mice were studied. Due to pharmacokinetic properties of PGM, namely its fast metabolism and excretion, special emphasis was on choosing the appropriate time for lymph node removal and duration of cell cultivation for each cytokine. PGM treatment in addition to OVA resulted in an increase of lymph node cellularity, stimulation of OVA-specific IFN-gamma and IL-4 production as well as of OVA-specific proliferative response. Results demonstrate that PGM stimulated both Th1 and Th2 subpopulations.

Failure To detect muramic acid in normal rat tissues but detection in cerebrospinal fluids from patients with Pneumococcal meningitis

Muramic acid serves as a marker for the presence of bacterial cell wall debris in mammalian tissues. There have been a number of controversial and sometimes conflicting results on assessing the levels of muramic acid in health and disease. The present report is the first to use the state-of-the art technique, gas chromatography-tandem mass spectrometry, to identify and quantify the levels of muramic acid in tissues. Muramic acid was not found in normal rat brain or spleen. However, when tissues were spiked with muramic acid, it was readily identified. The detection limit was <1 ng of muramic acid/100 mg (wet weight) of tissue. The levels of muramic acid reported in diseased human spleen and spleen of arthritic rats, previously injected with bacterial cell walls, were 100- to 1,000-fold higher. In the present study, muramic acid was also readily detected in the cerebrospinal fluid of patients with pneumococcal meningitis (6.8 to 3,900 ng of muramic acid/ml of cerebrospinal fluid). In summary, there can be an enormous difference in the levels of muramic acid found in different mammalian tissues and body fluids in health and disease. This report could have great impact in future studies assessing the role of bacterial cell wall remnants in the pathogenesis of certain human inflammatory diseases.

Selective activation of antitumor activity of macrophages by the delivery of muramyl dipeptide using a novel polynucleotide-based carrier recognized by scavenger receptors

We have shown that muramyl dipeptide (MDP) conjugated to a 10-mer polyguanylic acid (PolyG) is specifically internalized by macrophages through scavenger receptor (SCR)-mediated endocytosis. Macrophages activated by PolyG-MDP displayed about 20-fold higher cytotoxic activity against nonmacrophage tumor cells compared to that elicited by free MDP. The PolyG-MDP was found to trigger the secretion of higher levels of interleukin-6, interleukin-1alpha, TNF-alpha, and nitric oxide in comparison to free MDP. Addition of antibodies directed against IL-6 and TNF-alpha to macrophage culture completely abrogated the tumoricidal response of PolyG-MDP, indicating that these two cytokines are primarily responsible for bioefficacy. This general approach of PolyG as a vehicle may find wide application in the delivery of genes and antisense oligonucleotides to macrophages.

Absolute identification of muramic acid, at trace levels, in human septic synovial fluids in vivo and absence in aseptic fluids

This is the first report of a study employing the state-of-the-art technique of gas chromatography-tandem mass spectrometry for absolute identification of muramic acid (a marker for peptidoglycan) at trace levels in a human or animal body fluid or tissue. Daughter mass spectra of synovial fluid muramic acid peaks (> or = 30 ng/ml) were identical to those of pure muramic acid. Absolute chemical identification at this level represents a 1,000-fold increase in sensitivity over previous gas chromatography-mass spectrometry identifications. Muramic acid was positively identified in synovial fluids during infection and was eliminated over time but was absent from aseptic fluids.

Receptor mediated glycotargeting

Glycotargeting relies on carrier molecules possessing carbohydrates that are recognized and internalized by cell surface mammalian lectins. Numerous types of glycotargeting vehicles have been designed based on the covalent attachment of saccharides to proteins, polymers and other aglycones. These carriers have found their major applications in antiviral therapy, immunoactivation, enzyme replacement therapy and gene therapy. This review compared different types of glycotargeting agents and the lectins which have been successfully targeted to treat both model and human diseases. It may be concluded that the discovery of new mammalian lectins which endocytose their ligands will lead to the rapid development of new glycotargeting agents founded on the principles of carbohydrate-protein interactions.

T-cell-dependent immunobiological activity of a desmuramyl analog of muramyl dipeptide, adamantylamide dipeptide (AdDP), and D-isoglutamine

The present experiments demonstrate that, similar to immunomodulatory muramyl dipeptide, its desmuramyl analog adamantylamide dipeptide is able to induce mild and fully reversible paw edema in mice. This effect is an immune-related phenomenon depending on the activation of T-cell/macrophage interactions and on production of prostaglandins. Possible involvement of certain immunoregulatory/inflammatory cytokines (e.g. IL-1, IL-2) has been suggested. The most probable intrinsic moiety of the adamantylamide dipeptide molecule responsible for triggering the edema formation is obviously D-isoglutamine.

Pharmacokinetics of an immunomodulator peptidoglycan monomer in mice after intravenous administration

A 14C labeled low molecular weight immunomodulator, peptidoglycan monomer (14C-PGM), was injected intravenously (i.v.) into mice. At various time intervals thereafter (15 min-6 h), radioactivity in the urine, whole blood, plasma, kidneys, liver, spleen, lungs, intestines and the brain of the mice was determined. Shortly after injection, 14C-PGM was very rapidly excreted from the organism, so that 1 h following administration, 80% of the radioactivity was found in the urine (62% as unchanged PGM and the rest as the metabolites pentapeptide and disaccharide). At the same time, around 2% of the injected material was found in the blood. Six hours after injection, equal quantities were found in the intestines, liver and blood (0.5%), slightly less in the kidneys, lungs and spleen (0.2-0.3%) and the least quantity in the brain (0.04%). However, the dynamics of retention in the organs was evidently different. In the kidneys, lungs and spleen, radioactivity steadily decreased over the studied period. In the liver following an initial decrease, radioactivity remained the same 3 and 6 h after injection. On the other hand, in the intestines and brain PGM seemed to accumulate rather than disappear following i.v. administration. This fact should be considered when explaining different biological activities of low molecular weight bacterial peptidoglycans.

Influence of muramyl dipeptide on renal candidiasis in genetically distinct mice

Susceptible (DBA/2) and resistant (C57BL/6) mice were inoculated intravenously with Candida albicans to evaluate the effect of a four-day prophylaxis with muramyl dipeptide (MDP) on the renal burden of organisms during the first week after infection. In sham-treated DBA/2 mice injected with 8 x 10(4) candida cells, renal CFU (LOG10 +/- SEM) on days 1, 4 and 7 after infection were found to average 5.050 +/- 0.109, 4.882 +/- 0.133 and 5.482 +/- 0.245. In sham-treated C57BL/6 mice injected with 2 x 10(5) candida cells, renal CFU on days 1, 4 and 7 reached only 3.610 +/- 0.118, 3.404 +/- 0.107 and 4.176 +/- 0.580. MDP-treated DBA/2 mice achieved significant reduction in CFU of C. albicans on day 1 (1.3 log units) and day 4 (0.6 log unit), while MDP-treated C57BL/6 mice had significant reduction in CFU of C. albicans only on day 1 (0.6 log unit) after infection. Sham-treated mice of both strains had a 28.6 to 30% increase in kidney weights on day 4 only, a transient change not seen in MDP-treated mice. Histopathological examination on days 8, 15 and 21 after infection revealed a higher incidence of renal papillary necrosis in DBA/2 mice than C57BL/6 mice (approximately 70% vs 10%). The incidence of granulomas and of chronic interstitial inflammation was much higher in MDP-treated mice. We conclude that the genetic makeup of the host influences the potential effectiveness of MDP as a biological response modifier.

Rapid elimination of a synthetic adjuvant peptide from the circulation after systemic administration and absence of detectable natural muramyl peptides in normal serum at current analytical limits

Although it is clear that muramyl peptides are involved in sleep associated with bacterial infection, their role in normal physiological sleep is less certain. It has been speculated that "natural" muramyl peptides, derived from degraded gut flora, may pass into the bloodstream, where they play a role in normal sleep (M. Karnovsky, Fed. Proc. 45:2556-2560, 1986). Muramic acid serves as a chemical marker for muramyl peptides, since it is not synthesized by mammals. After injection of synthetic muramyl dipeptide in rabbits, muramic acid was readily detected (after release by acid hydrolysis) in the circulation; however, levels rapidly decreased. This was an important positive control in assessing circulating levels of natural muramyl peptides. Muramic acid was not found in normal serum (detection limit, approximately 500 pmol/ml), demonstrating the absence of appreciable amounts of circulating natural muramyl peptides. At this time we are unable to provide supportive evidence for Karnovsky's hypothesis.

Pharmacokinetic profile of the immunomodulating compound adamantylamide dipeptide (AdDP), a muramyl dipeptide derivative in mice

A pharmacokinetic profile of 14C-AdDP with uniformly labelled alanine was investigated. It was shown that the distribution phase after an i.v. administration is very short with a half-life of 2.1 min. The half-life of elimination phase after the i.v. administration is about 2.85 hours, that is longer than those of MDP and its derivatives. The total body clearance (30 ml/min/kg) is caused predominantly by metabolism of the compound. All the radioactivity found in urine in a 48 hours interval after a s.c. administration represents only 3.1% of the administered dose. Only a smaller part of the excreted radioactivity is formed by unmetabolised AdDP. The concentration curve after a s.c. administration is characterized by a very fast absorption with a half-life shorter than 1 minute. The distribution and elimination phases are prolonged (20 min, 11 hours respectively) in comparison with an i.v. injection. The decreased absolute bioavailability after a s.c. administration (65%) is probably not biologically significant because of a slower release of the compound from the site of the s.c. administration. A relatively very high radioactivity was found in liver, kidney, thymus, spleen and brain very soon which suggest a very good penetration into tissues. It is an agreement with the high apparent distribution volume of peripheral compartment and higher lipophilicity of AdDP as compared to MDP.

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.

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

We have investigated the ability of liposomes containing a lipophilic muramyl dipeptide, N-acetylmuramyl-L-alanyl-D-isoglutamine glycerol dipalmitate (MDP-GDP) to activate Kupffer cell tumoricidal activity in situ and to inhibit the growth of experimental hepatic micrometastases of tumor cell line H-59, a liver-homing variant of the Lewis lung carcinoma. Liposomes prepared from distearoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DSPC/DMPG) and containing MDP-GDP (1 mumol and 2 micrograms, respectively) were efficiently taken up by the liver after i.v. administration. A single i.v. injection of DSPC/DMPG liposomes containing MDP-GDP was capable of inducing Kupffer cell tumoricidal activity against H-59 tumor cells as measured in vitro. Control liposomes or 100 micrograms free MDP were ineffective in inducing Kupffer cell tumoricidal activity in situ. Two treatment regimens were evaluated in vivo: firstly, C57BL/6 mice were injected with tumor cell line H-59 and subsequently treated with multiple injections of liposomal MDP-GDP. Secondly, treatment with liposomal MDP-GDP was initiated prior to tumor cell injection and continued after tumor cell injection. The ability of liposomes containing MDP-GDP to reduce the number of hepatic micrometastases using the first protocol was related to the tumor cell inoculum, significant inhibition being observed at lower liver tumor burdens (less than 25 tumor nodules). Pretreatment of the mice prior to tumor cell challenge followed by treatment afterwards greatly enhanced the efficacy of liposomal MDP-GDP and brought about a highly significant inhibition of the growth of experimental metastases even at high liver tumor burdens (greater than 50 nodules).

Muramyl dipeptide bound to poly-L-lysine substituted with mannose and gluconoyl residues as macrophage activators

Poly-L-lysine modified with mannose derivatives, the residual cationic charges of which being neutralized by N-acylation, were synthesized and used as carriers of a macrophage activator (N-acetylmuramyl dipeptide, MDP). The influence of the acylating agent on the targeting efficiency was investigated: a hydrosolubilizing group such as a gluconoyl moiety led to very efficient carrier conjugates, while an acetyl group did not. The effect of sugar and acyl content of the polymers was assessed using these compounds as inhibitors of red blood cell agglutination by Concanavalin A. The binding and specific endocytosis of poly-L-lysine substituted with several mannose derivatives and gluconoyl residues (GlcAx-, Man(y)-PLK) have been determined by a quantitative flow cytometry analysis. MDP bound to these conjugates was much more efficient in vitro than free MDP in macrophage cytostasis assays.

Activation of murine Kupffer cell tumoricidal activity by liposomes containing lipophilic muramyl dipeptide

The ability of liposomes containing a lipophilic muramyl dipeptide, N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate, to induce Kupffer cell tumoricidal activity has been investigated. Liposomal N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate was 16-fold more potent than liposomal N-acetylmuramyl-L-alanyl-D-isoglutamine and 2,400-fold more potent than N-acetylmuramyl-L-alanyl-D-isoglutamine in inducing Kupffer cell tumoricidal activity in vitro. A single i.v. injection of liposomes containing N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate was capable of inducing Kupffer cell tumoricidal activity as measured against B16-melanoma cells after Kupffer cell isolation. Maximal cytotoxic activity was obtained with 1 microgram muramyl dipeptide-glycerol dipalmitate encapsulated within liposomes: doses of 10 or 100 micrograms inhibited tumoricidal activity. Kupffer cells from mice treated with liposomes containing N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate remained cytotoxic for at least 6 days after injection. Liposomal N-acetylmuramyl-L-alanyl-D-isoglutamine was significantly less potent than liposomal N-acetylmuramyl-L-alanyl-D-isoglutamine-glycerol dipalmitate in inducing Kupffer cell tumoricidal activity in situ. N-Acetylmuramyl-L-alanyl-D-isoglutamine was capable of inducing Kupffer cell tumoricidal activity in vitro: its failure to induce tumoricidal activity in situ at doses of 1,000 micrograms demonstrates the utility of liposomal carriers for the in vivo activation of Kupffer cells by muramyl dipeptides.

Anti-infectious activity of liposomal muramyl dipeptides in immunodeficient CBA/N mice

Two muramyl dipeptides, N-acetylmuramyl-L-alanyl-D-isoglutamine and its adjuvant-inactive isomer N-acetylmuramyl-D-alanyl-D-isoglutamine, were examined for their ability to protect mice carrying the CBA/N immune deficiency gene (xid) against lethal bacterial challenge. Prophylactic treatment with N-acetylmuramyl-L-alanyl-d-isoglutamine gave significant protection against Streptococcus pneumoniae, Salmonella typhimurium, and Salmonella enteritidis infection. N-Acetylmuramyl-D-alanyl-D-isoglutamine was unable to confer protection. Incorporation of the lipophilic glycerol dipalmitate derivatives of the two muramyl dipeptides within liposomal carriers resulted in a significant enhancement of anti-infectious activity, both with respect to number of survivors and length of survival. Liposomal muramyl dipeptides were 10- to 15-fold more potent than free muramyl dipeptide; enhanced potency was most evident with N-acetylmuramyl-D-alanyl-D-isoglutamine. Prophylactic treatment with liposomes containing the lipophilic muramyl dipeptides resulted in enhanced clearance of bacteria from the blood (greater than 3-fold increase in rate) when compared with that of hydrosoluble N-acetylmuramyl-L-alanyl-D-isoglutamine, indicating a correlation between reticuloendothelial stimulation and anti-infectious activity.

Comparative interaction of free and liposome-encapsulated nor-muramyl dipeptide or muramyl tripeptide phosphatidylethanolamine (3H-labelled) with human blood monocytes

The purpose of this study was to analyse on biochemical and functional levels the interaction of free and liposome-encapsulated nor-muramyl dipeptide (nor-MDP) or muramyl tripeptide phosphatidylethanolamine (MTP-PE) with human peripheral blood monocytes. The activation of tumoricidal properties in monocytes by free MTP-PE required approximately 40-fold less material than free nor-MDP. Encapsulation of either MTP-PE or nor-MDP within multilamellar liposomes (MLV) increased the efficiency of the immunomodulators for activation of monocytes. The initial interaction of free 3H-nor-MDP or 3H-MTP-PE with monocytes was influenced by lipophilic derivatization, but neither derivatives exhibited characteristics of specific binding to the monocyte surface. The encapsulation of 3H-nor-MDP or 3H-MTP-PE within MLV increased uptake of both compounds by monocytes. The metabolic fate of MLV-entrapped 3H-nor-MDP was unaltered, but liposome encapsulation retarded the metabolism of 3H-MTP-PE. Collectively, the data suggest that the activation of monocytes by muramyl peptides results from an intracellular interaction which can be modulated by both lipophilic derivatization and/or liposome-encapsulation of this immunomodulator.

Relationship of metabolism and immunostimulating activity of peptidoglycan monomer in mice after three different routes of administration

[14C] Peptidoglycan monomer, GlcNAc-MurNAc-L-Ala-D-isoglutamine-meso-diaminopimelic acid (omega NH2)-D-Ala-D-Ala (PGM) was administered to mice by the intravenous (i.v.), subcutaneous (s.c.) or peroral (p.o.) routes. The data on distribution of radioactivity and excretion of radioactive products, as well as the data on immunostimulating effects are presented on the comparative basis for PGM administered by three different routes. When injected i.v. or s.c., the major part of applied radioactivity was found excreted in urine, partly as unchanged original compound and partly as the corresponding pentapeptide, L-Ala-D-isoglutamine-meso-diaminopimelic acid (omega NH2)-D-Ala-D-Ala. If administered p.o., the major part of the radioactivity was retained in the stomach and intestinal tract for several hours. The drop in radioactivity in these organs was followed by exhalation of 14CO2 thus indicating extensive degradation of the original molecule. PGM stimulates the humoral immune response to sheep red blood cells in mice if administered i.v. or s.c., but is completely inactive if administered p.o.. Thus, absence of immunostimulating activity following p.o. administration might be explained by extensive metabolic degradation of peptidoglycan monomer.

Degradation of muramyl dipeptide by mammalian serum

Muramyl dipeptide, N-acetylmuramyl-L-alanine-D-isoglutamine (MDP), is the minimal biologically active subunit of bacterial peptidoglycan and elicits an acute inflammation in vivo. We now report that MDP is degraded by normal rat serum into its constituents, N-acetylmuramic acid and L-alanine-D- isoglutamine. The dipeptide is further degraded into its components L-alanine and D-isoglutamine. These results may help to explain how inflammation elicited by MDP is terminated in vivo.