By Wahid Khan & Ashutosh Kumar
The authors are with National Institute of Pharmaceutical Education & Research (NIPER), Hyderabad
Macrophages are the major differentiating cells of the mononuclear phagocyte lineage. They play a dual role in infection:
(a) they can trap, ingest and kill microorganisms as well as help in eliminating the threat of infection,
(b) but some pathogens adapt to survive within macrophages and exploit their intracellular position.
This intracellular position provides a protected environment to pathogen, hidden from the immune system and favours its replication.
Tropical diseases due to this kind of microbial invasion include tuberculosis (caused by Mycobacterium tuberculosis) and leishmaniasis (caused by Leishmania species). India contributes around 50 per cent of global burden of visceral leishmaniasis and 25 per cent of global burden of tuberculosis and accounts for maximum patients for both these diseases.
The major limiting factors are therapeutic efficacy and dose dependent toxicity of currently used chemotherapeutic agents. When drugs are injected in soluble form by using conventional dosage forms, only a small fraction of it reaches microenvironment of the intracellular parasite and most of it stays in the host systemic circulation. Therefore, pharmaceutical scientists have endeavours towards development of new carrier systems. In such instances, the concept of selective delivery of drugs targeting to macrophages can be a futuristic strategy.
The sole objective behind the design of such drug delivery systems should be to improve the aim of drugs with concomitant minimisation in the incidence of associated adverse effects. For maximum parasite eradication and to reduce in the toxicity, drug was intended to be released in to macrophages. For the past few decades, there has been a considerable research interest in the area of drug delivery using nanomedicine and nanoformulations. Various lipid and polymeric carriers (liposomes, nanoparticles, microspheres, niosomes and polymer drug conjugates, etc) are in extensive evaluation for transferring drugs directly to macrophages.
Liposomes
Liposomes are biodegradable, non-immunogenic, microscopic or submicroscopic vesicle of 10 nm to 20 mm diameter made of phospholipids. Liposomes can be broadly classified into four types based on their composition, structure, and in vivo applications:
- conventional liposomes, it is further sub-classified into three types: small unilamellar liposomes (single bilayer, 10nm-50nm), large unilamellar liposomes (single bilayer, 50nm-1,000nm) and large multilamellar liposomes (many layers, 100nm-20mm)
- long circulating liposomes, also known as stealth liposomes;
- Targeting liposomes, liposomes with other targeting moieties;
- multivesicular liposomes. Depending upon the chemical composition liposomes can have positive, negative or neutral surface charge. They can carry both hydrophilic (in core) and hydrophobic drug (in the corona).
Liposomes allows high drug/vehicle ratio and they are accumulated specifically within macrophages by phagocytosis. Phagocytosis of liposomes take place in the sequence as adsorption on to phagocytic cells, internalisation of vesicle by energy dependent mechanism, fusion of endocytosed vesicle with lysosome, degradation of liposome in the lysosome, release of drug encapsulated in liposomes, nearly same internalisation was observed with other nanoformulations.
One such product, AmBisome (liposomal amphotericin B) is the best available treatment for the visceral leishmaniasis, which is a good tradeoff between activity and toxicity. AmBisome is also considered as first choice for treating patients who are unresponsive to other drugs. But high production costs make it unaffordable for patients in many of developing poor countries where the requirement is more.
Microspheres
Over the last 25 years, research on microspheres for drug delivery has surfaced to attain the unmet needs in targeted drug delivery. Microspheres are defined as homogeneous, monolithic particles in the size range of about 1 to 1000 micron and are widely used as drug carrier for controlled release formulations. Microspheres which offer stable, relatively cheap option were selected to be used as delivery system. They have greater encapsulation efficiency and greater stability. Microparticles are made from biocompatible and biodegradable materials such as polymers, either natural (gelatin, albumin) or synthetic (polylactides, polyalkylcyanoacrylates), or solid lipids.
Other more commonly used synthetic polymers include polyamides, polyamino acids, polyorthoesters, polyurathanes, polyacrylamide, etc. Microspheres of size below 6 μm are engulfed by the macrophages. Chances of phagocytosis are maximum when the size is 1-2 μm. As that of liposomes, size, surface property, composition, concentration and hydrophilicity of microspheres play a significant role in the uptake by macrophages. Hydrophobic microspheres are more susceptible to the phagocytosis than hydrophilic microspheres. The extent of phagocytosis can be improved by coating the particle surface with opsonic materials.
Nanoparticles and nanosuspension
Nanoparticles range between 10-1000 nm and can be of polymeric or lipid nature. Nanoparticles for the purpose of drug delivery are defined monolithic particles in which the drug is adsorbed, dissolved, or dispersed throughout the matrix and nanocapsules in which the drug is confined to an aqueous or oily core surrounded by a shell-like wall. Drugs can be either integrated in the matrix or attached to the surface.
Nanoparticle based drug delivery system has demonstrated promising results macrophage delivery over the last decade, consisting of various synthetic biodegradable materials, such as natural or synthetic polymers, lipids, phospholipids and even metals. At cellular level the nanoparticles are endocytosed/ phagocytosed by cells, which results in internalisation of the encapsulated drug.
Nanoparticles constitute a versatile drug delivery system which can potentially overcome physiological barriers, and guide drugs to specific cells or intracellular compartments; either by passive or ligand mediated targeting approaches. The versatility of formulation, colloidal size, biocompatibility and sustained release properties of nanoparticles have already been accepted with growing interest for a wide range of applications. Hydrophobicity and size are crucial factors governing uptake of nanoparticles by macrophages. Nanoparticles are rapidly cleared by macrophages after IV administration.
Polymer drug conjugate and lipoproteins
As a potential alternate, ‘prodrug approach’ using water-soluble polymer can be used to deliver drugs. According to this approach, therapeutic compounds are modified to form a ‘prodrug’, which is inactive during the delivery to the site of action and is converted into an active drug at the target organ, tissues or cells with a predetermined time-profile. The ultimate goal is to construct an ideal drug as a ‘magic bullet’, which will effectively kill only targeted cells or microorganisms, suppress or activate just the targeted processes without damaging normal healthy cells or altering their natural functions.
Polymer drug conjugate/prodrugs are chemical entity of an active parent drug with altered physico-chemical properties like increased aqueous solubility, enhanced bio-distribution while retaining the inherent pharmacological properties of the drug. The utilisation of prodrugs, to a certain extent, allows for the preservation of activity of a drug and targets its release to certain cells or their organelles. Solubility of the sparingly soluble drugs can be increased by forming drug hydrophilic polymer conjugate. These conjugates can be endocytosed by the macrophage specific receptors. Arabinogalactan, hydroxypropylmethyl cellulose, or dextrans are some of polymers which are used to make these carrier systems. Dendrimers can also be used to conjugate drugs. Studies carried out in macrophages showed that the targeted conjugate had a high uptake as compared to the non-targeted conjugates.
Conclusions
Despite significant developments in various fields of science, Tropical Diseases remains a serious public health problem in India. The treatment is extremely unsatisfactory; drugs are very costly and toxic. Considering the burden of disease in poorer sections of society and widespread emergence of drug resistance, better drug therapy is required. New formulations, therapeutic switching of the established drugs and the discovery of new drugs should be implemented to improve chemotherapy. Several strategies are there to deliver drugs to the required site either by passive or receptor-mediated active targeting in the form of different novel dosage forms. Each system has its own advantages and disadvantages and should be optimised and selected according to drug properties and need for better therapeutics.
Acknowledgement
The authors would like to acknowledge Project Director Dr Ahmed Kamal and Registrar Prof N Satyanarayna, NIPER-H, for support in the preparation of this article.
Further Readings
- W Khan, N Kumar. Drug targeting to macrophages using paromomycin-loaded albumin microspheres for treatment of visceral leishmaniasis: an in vitro evaluation. J Drug Target 2011, 19, 239.
- S Das, W Khan, S Mohsin, N Kumar. Miltefosine loaded albumin microparticles for treatment of visceral leishmaniasis: formulation development and in vitro evaluation. Polym Adv Technol 2011, 22, 172.
- A Nan, SL Croft, V Yardley, H Ghandehari. Targetable water-soluble polymer-drug conjugates for the treatment of visceral leishmaniasis. J Control Release 2004, 94, 115.
- F Ahsan, IP Rivas, MA Khan, AI, Torres Suarez, Targeting to macrophages: role of physicochemical properties of particulate carriers-liposomes and microspheres-on the phagocytosis by macrophages. J Control Rel 2002. 79, 29-40.
- F Chellat, Y Merhi, A Moreau, L Yahia. Therapeutic potential of nanoparticulate systems for macrophage targeting. Biomaterials 2005, 26, 7260
