Synthesis, characterization and applications of polymer based conjugate materials for infectious diseases of poverty

Abstract

The major disease burden for the majority of the world’s population is due to infectious diseases. The most prevalent are HIV/AIDS, tuberculosis, lower respiratory tract infections, diarrhoeal diseases and malaria. In particular, malaria and tuberculosis have not benefited much from new technological developments in disease management. Most of the drugs used are several decades old and have significant toxicity profiles which impact patient compliance. New potential drugs being discovered stumble on the road to the clinic because of solubility issues. Many just end up being shelved. It is estimated that over 70% of new chemical entities have poor solubility issues. On the other hand, HIV/AIDS remains a major health problem worldwide, as currently there is no cure available. Chronic intake of highly active antiretroviral treatment (HAART) is compulsory to control HIV infection and any non-adherence leads to a quick increase in the viral load. Poor targeting ability of antiretrovirals (ARVs) to latent sites of infection is the main reason for the relapse. Nanomedicines have significantly improved the clinical management of deadly diseases like cancer. Conventional drugs show improved pharmacological indices when designed as nanomedicines. It improves solubility, absorption, clinical performance, and reduces the amount of drug needed to achieve the desired therapeutic effect. The resultant effect is improved solubility and reduced toxicity. The most common method to design nanomedicines is through physical entrapment or encapsulation in polymeric carriers. Today, however, modern delivery systems are being designed by chemical synthesis. Either the drugs are chemically linked to the polymeric carriers or the polymers are chemically derivatized to be more efficient at encapsulating the active agents. In this project, we report on our attempts to chemically modify polymers with active drugs to synthesize smart macromolecular pro-drugs or produce more efficient drug encapsulation systems. The work presented in this project is outlined in the following six chapters. Chapter 1 contains the literature review of the major infectious diseases; namely HIV/AIDS, tuberculosis and malaria; what has been done to increase patient compliance or success of available drugs i.e (to reduce the viral load or attempted to cure the virus with the ARVs in the field of HIV/AIDS, or administration of combination therapy in anti-tuberculosis/malarial field to minimize drug resistance and increase therapeutic success). The use of nanomedicine to ameliorate problems associated with treatment regime for infectious diseases is discussed. Methods for nanoencapsulation or inclusion of the existing drugs within the approved materials is described in detail. A concise general introduction to polymer therapeutics is discussed, followed by a review of polymers that are normally used in polymer-drug conjugation; highlighting advantages and disadvantages of each. Finally a clinical perspective on the use of polymer-drug conjugation for infectious diseases is outlined. Chapter 2 introduces the problems associated with the current ARVs used in the treatment of HIV/AIDS and emphasizes the need to target latent sites of infection using aptamer technology. Aptamers were subsequently conjugated to polyethylene glycol (PEG) using carbodiimide chemistry. The TZM-bl neutralization assay and in vitro stability in human breast milk studies showed that aptamers maintained their binding integrity after pegylation and were more stable than the parent aptamers. Finally the conjugated aptamers were nanoencapsulated into poly(epsilon-caprolactone) [PCL] nanoparticles using a double emulsion method. Nanoparticles of less than 150 nm were produced with a higher surface charge, showing that the nanoparticles were stable. The in vitro binding assay using electrochemical methods showed that the nanoparticles coated with PEGylated RNA aptamers had higher affinity and specifity to HIV-1 gp 120. The overall results demonstrated that these nanoparticles could be used in HIV drug delivery applications to help minimize changes associated with ARVs. Chapter 3 describes nanomedicinal formulations of the anti-TB drug moxifloxacin (Mox). Mox is a relatively hydrophilic drug. The target pathological site where the Mycobacterium tuberculosis resides is a lipid dense granuloma in the lungs. Hence, a large dose will have to be administered to deliver an adequate therapeutic dose i.e. 400 mg is required for Mox. Mox was covalently conjugated into PEG via a releasable amide bond. Similarly a hybrid system was formed by nanoencapsulating the PEG-Mox conjugate into PCL nanoparticles using double emulsion method. The system constitutes the PCL, which is envisaged to increase the hydrophobicity of the PEG-Mox conjugate. PEG-Mox conjugates and PCL-Mox nanoparticles were found to be hemocompatible, inducing only minimal hemolysis. Mox was more toxic than the PEG-Mox conjugate and PCL-Mox nanoparticles. In vitro stability in human plasma showed that PCL-Mox nanoparticles were stable for over 72 hrs. Data obtained emphasizes that PCL nanoparticles could be used as a drug delivery system to minimize the high toxicity of TB drugs Chapter 4 establishes the conjugation of the hydrophobic drug, lumefantrine [Lumf] to water soluble polymers. Lumf is insoluble in water with an octanol-water partition coefficient (logp) of 8.34. As a result a series of Lumf prodrug conjugates were synthesized using two different polymers (polyethylene glycol and p-NAM-stat-p-AA). Average particle size below 200 nm was achieved and PDI values were always below 0.2, which is an indication of the relatively homogeneous size distribution achieved with carbodiimide chemistry. We have for the first time, by applying the polymer therapeutics techniques, synthesized a polymer-drug conjugate of Lumf which has increased the solubility of the drug more than 103 times. Chapter 5 gives the conclusions of the experimental chapters. Through the three different experimental chapters, we have demonstrated that polymer based drug conjugates can be used to address different issues: (1) drug delivery through coating nanoparticles with appropriate aptamers, (2) drug toxicity through encapsulation of a toxic drug in a heamocompatible nanoparticle and (3) greatly improved aqueous solubility of a hydrophobic drug. Furthermore, while there has been much excellent work using polymer based drug conjugates in cancer, we have explored the approach to tackle different problems related to three different infectious diseases of poverty, namely HIV, TB and malaria.