Pure drug and polymer based nanotechnologies for the improved solubility, stability, bioavailability and targeting of anti-HIV drugs��� Puneet Sharma, Sanjay Garg ��� School of Pharmacy, The University of Auckland, Auckland, New Zealand a b s t r a c t a r t i c l e i n f o Article history: Received 14 June 2009 Accepted 14 September 2009 Available online 18 November 2009 Keywords: HIV Polymer Nanoparticle Nanocrystal Polymeric micelle Dendrimer The impact of human immunodeficiency virus (HIV) infection has been devastating with nearly 7400 new infections every day. Although, the advent of highly active antiretroviral therapy (HAART) has made a tremendous contribution in reducing the morbidity and mortality in developed countries, the situation in developing countries is still grim with millions of people being infected by this disease. The new advancements in the field of nanotechnology based drug delivery systems hold promise to improve the situation. These nanoscale systems have been successfully employed in other diseases such as cancer, and therefore, we now have a better understanding of the practicalities and technicalities associated with their clinical development. Nanotechnology based approaches offer some unique opportunities specifically for the improvement of water solubility, stability, bioavailability and targeting of antiretroviral drugs. This review presents discussion on the contribution of pure drug and polymer based nanotechnologies for the delivery anti-HIV drugs. �� 2009 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 2. Pure drug nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 3. Polymer based nanotechnologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 3.1. Polymeric micelles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 3.2. Polymeric nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496 3.3. Dendrimers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 4. Conclusion and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 1. Introduction Human immunodeficiency virus (HIV) related acquired immune deficiency syndrome (AIDS) has claimed over 25 million lives since its discovery in 1981. Based on the profound knowledge gained about the HIV replication cycle, several drug targets have been identified over the years and effective treatment options are currently available [1]. The current clinical therapy, known as ���highly active antiretroviral treatment��� or HAART, is considered as one of the most significant advances in the field of HIV therapy [2]. Since mid 1990s, HAART has made a remarkable contribution towards reducing the mortality in patients. Recommended HAART regimens include at least three actives [3]. Since majority of antiretrovirals are administered orally, their adequate systemic absorption from gastrointestinal (GI) tract is a prerequisite for successful therapy. Amidon et al. [4] suggested a Biopharmaceutic Classification System (BCS) that identifies the solubility and permeability of drug substances as markers for their oral bioavailability. Following BCS guidelines, a drug substance is considered ���highly soluble��� when its highest dose strength solubilizes in 250 ml or less of aqueous media over a pH range of 1.0���7.5 at 37 ��C [5]. Likewise, a drug substance is considered ���highly permeable��� when Advanced Drug Delivery Reviews 62 (2010) 491���502 ��� This review is part of the Advanced Drug Delivery Reviews theme issue on ���Nanotechnology Solutions for Infectious Diseases in Developing Nations���. ��� Corresponding author. School of Pharmacy, The University of Auckland, Private Bag 92019, Auckland, New Zealand. Tel.: +64 9 373 7599x82836 fax: +64 9 367 7192. E-mail address: s.garg@auckland.ac.nz (S. Garg). 0169-409X/$ ��� see front matter �� 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.addr.2009.11.019 Contents lists available at ScienceDirect Advanced Drug Delivery Reviews journal homepage: www.elsevier.com/locate/addr

the extent of the intestinal absorption (parent drug plus metabolites) in humans is determined to be ���90% of an administered dose based on a mass balance determination or in comparison to an intravenous (i.v.) reference dose [5]. However, there are other factors such as effect of efflux and absorptive transporters, intestinal metabolizing enzymes (CYP34A and UDP-glucuronosyltransferases (UGTs)) and food that have to be taken into consideration for the appropriate estimation of the bioavailability [6]. Considering properties such as highest dose strength, dose number, solubility and log P, Kasim et al. [7] have classified twelve antiretrovirals (12th edition of World Health Organization Essential Core Drug List) in different BCS classes (Table 1). Out of these twelve antiretrovirals, nine have either solubility (BCS Class 2) or permeability (BCS Class 3) related issues. Consequently, oral absorption and, in turn, bioavailability is variable (Table 2) and mainly dependent on (i) stability in stomach acid conditions, (ii) aqueous solubility and dissolution from dosage form and (iii) permeability through the intestinal membrane. Most of the protease inhibitors (Pis) have high molecular weight (N500 Da), pH dependent solubility (high solubility at acidic pH) and high log P (high lipophilicity) (Table 3), and all these properties adversely affect the oral bioavailability [8]. In addition, they are substrates of P- glycoprotein efflux transporters. Similarly, although the majority of nucleoside reverse transcriptase inhibitors (NRTIs) show good systemic absorption (N70% at clinically relevant doses), didanosine (ddI) and zidovudine (AZT) show variable bioavailability. ddI is unstable at acidic pH and is subjected to first-pass metabolism which may be responsible for its lower bioavailability. AZT also undergoes first-pass metabolism and additionally shows dose dependent effect on bioavailability. HAART is a lifelong necessity and any non-compliance leads to a rapid increase in the viral load [2]. The reason for this relapse is related to the poor targeting ability of the antiretroviral agent to the latent sites of infection [9]. Another major limitation of the current HIV treatment is linked to the short residence time and resulting low concentration of antiretroviral drugs at certain inaccessible viral reservoir sites such as lymphatic system, macrophages, lymphocyte, central nervous system (CNS) and lungs [10]. Thus, administration of higher doses is required for prolonged duration to eradicate the virus. This prolonged therapy is often associated with the emergence of resistant HIV strains. In addition, there are major issues of adverse drug reaction and drug���drug interaction associated with antiretro- viral therapy. Although newer regimens in HAART have significantly lower toxicity, modulation of lipid and glucose metabolism is still a major issue [2]. After chronic treatment even moderate toxicity may lead to serious complications. The resulting treatment failure not only affects the quality of patient's life but also significantly adds to the economic burden of the health care system. In the context of oral drug delivery, the important characteristics of a molecule that needs to be considered for positive anti-HIV effects are (i) solubility and ionization, (ii) lipophilicity and permeability, (iii) stability in biological fluids, (iv) gastrointestinal metabolism and (v) viral reservoir targeting. When these properties are unfavorable for drug development, alternative processing and formulation specific approaches can be employed to attain maximum therapeutic gains. The nanometer size and high surface area to volume ratio which affect the pharmacokinetics and biodistribution of the associated drug molecule are main features of nanotechnology based drug delivery systems. The nanotechnology based approaches discussed in this review for the delivery of anti-HIV drugs include pure drug nanoparticles, polymeric micelles, dendrimers and polymeric nano- particles. Several reviews are available on various pharmaceutical aspects of these nanoparticulate systems such as preparation methods, physicochemical properties, toxicity and others ([11���17]). This review presents information pertaining to their applicability specifically for anti-HIV drugs. 2. Pure drug nanoparticles In a pure drug nanoparticle formulation, submicron size particles (mean particle size b1 ��m and 99th percentile particle size of b5 ��m) of drugs are stabilized in aqueous medium with GRAS (generally recognized as safe) stabilizers. Such formulation can be used for drugs with properties such as insolubility in both water and oil, high melting point, high log P and high dose [18]. Following Noyes���Whitney equation, progressive size reduction of the drug particles leads to an increase in the surface area resulting in an increased dissolution rate. Additionally, particle size reduction results in the decrease of the diffusion layer thickness surrounding the particles [19] and an increased concentration gradient between the surface of the particle and bulk solution, which facilitates particle dissolution by increasing dissolution velocity. Therefore, nanosizing is a suitable approach for increasing bioavailability of those drugs where dissolution is the rate limiting step in systemic absorption. Van Eerdenbrugh et al. investigated the dissolution and in vitro absorption of a poorly water soluble non-nucleoside reverse tran- scriptase inhibitor (NNRTI), loviride (water solubility 0.1 mg/l), after nanonization [20]. In addition to poor water solubility, loviride has a high melting point (225 ��C) and poor solubility in oils. Such physicochemical properties make loviride a ���brick-dust��� molecule with low oral bioavailability. Media milling led to mean particle sizes Table 1 Antiretroviral drugs in the WHO essential core drug list classified according to BCS (adapted from [7]). Drug Highest dose strength (mg) Solubility (mg/ml) Dose number (Do)a CLogP Log P pKa (s) BCS class Log P-based CLogP-based Indinavir sulfate 400 1000 0.0016 3.68 2.49 1 1 Nelfinavir mesylate 250 4.5 0.22 5.84 4.62 ���1.2 1 1 Saquinavir mesylate 200 2.22 0.36 4.73 2.73 1 1 Efavirenz 200 0.01 80 4.95 3.68 10.2 2 2 Lopinavir (with ritonavir) 133.3 0.01 53.3 6.1 4.56 2 2 Nevirapine 200 0.1 8 2.42 2.05 2.8 2 2 Ritonavir 100 0.01 40 4.94 5.98 2 2 Abacavir sulfate 300 77 0.016 0.58 0.22 5.01 3 3 Didanosine 200 27.3 0.03 ���1.92 ���1.1 9.12 3 3 Lamivudine 150 70 0.0086 ���1.46 0.06 3 3 Stavudine 40 83 0.002 ���0.73 ���0.47 3 3 Zidovudine 300 20.1 0.06 0.04 3 Classification criteria: dose number���1=high solubility and N1=poor solubility. Estimated log P and CLogP values���1.72 and 1.35=high permeability and b1.72 and 1.35=low permeability. BCS classification: class 1 high solubility, high permeability class 2 low solubility, high permeability class 3 high solubility, low permeability class 4 low solubility, low permeability. a Do is the ratio of drug concentration in the administered volume (250 ml) to the saturation solubility of the drug in water and calculated as Do=(Mo/Vo)/Cs where Mo is the highest dose strength (mg), Cs is the solubility (mg/ml), and Vo=250 ml. 492 P. Sharma, S. Garg / Advanced Drug Delivery Reviews 62 (2010) 491���502