4. Transcription

When the host cell receives a signal to become active, the provirus uses a host enzyme called RNA polymerase to create copies of the HIV genomic material, as well as shorter strands of RNA called messenger RNA (mRNA). The mRNA is used as a blueprint to make long chains of HIV proteins.

5. Assembly

An HIV enzyme called protease cuts the long chains of HIV proteins into smaller individual proteins. As the smaller HIV proteins come together with copies of HIV's RNA genetic material, a new virus particle is assembled.

6. Budding

The newly assembled virus pushes out ("buds") from the host cell. During budding, the new virus steals part of the cell's outer envelope. This envelope, which acts as a covering, is studded with protein/sugar combinations called HIV glycoprotein. These HIV glycoproteins are necessary for the virus to bind CD4 and co- receptors. The new copies of HIV can now move on to infect other cells.

Current Status of Anti-AIDS Agents

Currently, available Anti-AIDS agents include reverse transcriptase inhibitors, protease inhibitors and integrase inhibitors which act on enzyme reverse transcriptase, protease and integrase, respectively. Fusion inhibitors are also used which prevent binding of HIV to the CD4+ cells.

Within 25 years, 25 anti-AIDS drugs have been approved for clinical use in the treatment of HIV infections: seven nucleoside reverse transcriptase inhibitors (NRTIs): zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir and emtricitabine; one nucleotide reverse transcriptase inhibitor (NtRTI): tenofovir [in its oral prodrug form: tenofovir disoproxil fumarate (TDF)]; four non-nucleoside reverse transcriptase inhibitors (NNRTIs): nevirapine, delavirdine, efavirenz and etravirine; ten protease inhibitors (PIs): saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, lopinavir, atazanavir, fosamprenavir, tipranavir and darunavir; one fusion inhibitor (FI): enfuvirtide; one co-receptor inhibitor (CRI): maraviroc and one integrase inhibitor (INI): raltegravir. These compounds are used in various drug combination (some at fixed dose) regimens so as to achieve the highest possible benefit and tolerability, and to diminish the risk of virus-drug resistance development.⁵

Maraviroc is the first CCR5 antagonist drug approved for clinical use and represents a milestone in the development of new treatments against HIV infection. Maraviroc is a novel drug and different from the rest of the antiretrovirals due to the special characteristics of its mechanism of action and is also the first antiretroviral directed towards a cell target.⁶

The fact that the HIV is endowed with unique viral enzymes and genes that are required for replication provide the attractive target for drug design. The current chemotherapeutic strategies still suffer from issues of cost, patient compliance, deleterious acute and chronic side effects, emerging single and multidrug resistance, and generalized treatment and economic issues. Even the best antiretroviral therapeutic strategy, highly active antiretroviral therapy (HAART), falls short of completely suppressing HIV replication. Therefore, expansion of current therapeutic options by discovering new antiretrovirals and targets as well as development of known inhibitors will be critical in the coming years.^7,
8

Novel targets for Anti-AIDS Agents

The study of HIV structure, genome and its life cycle, at molecular level, has revealed a many exciting targets for treatment of Acquired immunodeficiency syndrome (AIDS).

1. Inhibitors that directly or indirectly target Env ^9,10,11

Env is a proviral genome which mediates entry of HIV into target cells, through two glycoproteins (GP¹²⁰ and GP⁴¹), via a multistep process that presents three distinct targets for inhibition by viral and cellular-specific agents.

First, attachment of virions to the cell surface via nonspecific interactions and CD4 binding can be blocked by inhibitors that include cyanovirin-N, cyclotriazadisulfonamide analogues, PRO 2000, TNX 355, zintevir, FP-21399 and PRO 542. In addition, BMS 806 can block CD4-induced conformational changes.

Secondly, Env interactions with the co-receptor molecules can be targeted by chemokine antagonist acting as HIV entry inhibitors which interact both with the CCR5 antagonists including SCH-D, maraviroc (UK 427857) TAK-779, TAK-220, SCH-C, SCH-D, E913, AK-602,NSC 651016 and aplaviroc (GW 873140), and the CXCR4 antagonist AMD 070, AMD3100, AMD3465, ALX40-4C, T22, T134 and T140. Also, the HIV-1 Tat protein has been described as a "natural" CXCR4 antagonist with anti-HIV-1 activity.

Thirdly, fusion of viral and cellular membranes can be inhibited by peptides such as enfuvirtide and tifuvirtide (T 1249), pentafuside, T-20 .The development of entry inhibitors has been rapid, with an increasing number entering clinical trials. Moreover, some entry inhibitors are also being evaluated as candidate microbicides to prevent mucosal transmission of HIV.

2. Integrase Inhibitors ^9,10,11

The integrase enzyme facilitates the integration of viral DNA into the host cell genome. The uniqueness and specificity of this reaction makes integrase an attractive drug target. However, integrase inhibitors have been slow to reach clinical development, although recent contenders, including L-870810, L-731, L-988 and S-1360 show promise.

3. Aturation Inhibitors ^9,10,11

Inhibitors that target the substrate of protease enzymes results in emergence of non-infectious virus. PA 457 is under investigation.

4. Zinc finger Inhibitors ^12,13

The inner core of HIV is called the nucleocapsid. It is held together by structures called "zinc fingers." Zinc finger inhibitors (or zinc ejectors) are drugs that can break apart these structures and prevent the virus from functioning. As nucleocapsid core cannot mutate very easily, so a drug that works against zinc fingers might be effective for a long time. Blocking zinc fingers means that HIV makes copies of itself that do not work and cannot infect new cells. 2, 2’-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA) are zinc finger inhibitors which are under clinical trial.

5. Transcription inhibitors ¹⁴

Transcription inhibitors inhibit the viral mRNA and hence, inhibit the transactivation process. The peptoid CGP64222, fluroquinolone k-12, streptomyces product Em 2487 are under investigation.

6. Gene Therapy

Anti-AIDS drugs designed to interfere with obligatory utilization of certain host cell factors by virus are less likely to encounter development of resistant strains than drugs directed against viral components. Several cellular genes required for productive infection by HIV were identified by the use of genetic suppressor element (GSE) technology as potential targets for anti-AIDS drug development.¹⁵

Gene therapy of HIV-1 represents one such treatment and several strategies are currently under development.¹⁶

a. The naturally-occurring RNA interference (RNAi) pathway represents a powerful tool for the sequence-specific post-transcriptional silencing of gene expression. By exploiting the endogenous mammalian RNAi pathway, several expression-based strategies have been developed to inhibit human immunodeficiency virus (HIV) gene expression and replication. This approach potentially has utility as a protective 'therapeutic vaccine' of virus-susceptible lymphocytes. Particular attention is given to advances in combinatorial gene expression systems that prevent the emergence of RNAi-resistant virus by simultaneously targeting multiple HIV targets. Researchers from countries including Russia are developing the artificial RNA-interference system. It is non-injurious to the patient and, due to high specificity of action, does not damage its own RNA in cells infected by the virus.

To fight against HIV, Russian biologists have created three genetic structures. These structures contain short nucleotide sequences that find the most conservative molecules among all RNA molecules, that is, sequences that do not change quickly and are important to the virus. These sequences are then "damaged”. These approaches form the basis for a number of promising ongoing and future clinical trials aimed at providing an effective, safe and prolonged single-intervention therapy for HIV/AIDS.¹⁷

b. A mutant tRNA has been developed for use against HIV-1 integration. This novel tRNA selectively interrupts viral integration into the genome by targeting key steps in this pathway. Most other contemplated therapeutic approaches act after the virus has integrated into the host cell's DNA and may be less effective once infection is established. A therapeutic strategy would entail introduction of the mutant tRNA into cells typically targeted by HIV-1. One approach would use a viral vector to infect the target cells and to insert genes that code for the mutant tRNA. Once the mutated tRNA is in the cell, a number of very specific actions could potentially impair viral integration. The mutant tRNA has high affinity to the HIV-1 reverse transcriptase mRNA, making this an ideal therapeutic approach with low toxicity.¹⁸

Several other products are developed to interfere with genes used by HIV which are as follow. (www.aidsinfonet.org, Fact Sheet Number 470)

 HGTV43 by Enzo Biochem is an “antisense” therapy designed to produce CD4 cells (T-cells) that resist infection by HIV. It is in Phase I trials.

 M87o by EUFETS AG is a gene therapy that makes CD4 cells resists infection by HIV. It is being studied in a Phase I trial.

 Mifepristone (VGX410, also known as RU486) by Viral Genomix interferes with the viral protein vpr. It is in a Phase I/II trial.

 Modified CD4 and CD8 cells by Cell Genesys are genetically modified to block attachment by HIV.

 RRz2 by Johnson and Johnson is a ribozyme that attacks HIV’s tat gene. It is in Phase II trials.

 SB-728-T by Sangamo BioSciences is used to genetically modify a patient's own CD4 cells to make them immune to HIV infection. It is in a Phase I trial.

 VRX496 by VIRxSYS is in Phase II trials. It appears to bind to the RNA (genetic code) of HIV and disrupt it.

7. Capsid protein and cyclophilin ¹⁹

HIV-1 capsid (CA) protein and human cyclophilin A (CypA) play important roles in assembly and disassembly processes, which make them attractive targets of high priority. Inhibitors that target CA or CypA have been mainly divided into three classes:

(1) Compounds that specifically block capsid protein formation

(2) Compounds that directly bind to the capsid and inhibit its assembly.

(3) Compounds that bind to Cyp A and possibly inhibit the disassembly of capsid conical cores.

Here, we give an overview of HIV-1 CA protein and Cyp A as new targets for potential anti-AIDS therapeutic agents.

8. Antibodies that targets HIV gp120 ²⁰

A novel small molecule, designated as ARM-H, has the potential to interfere with HIV survival through two mechanisms,

(1) By recruiting antibodies to gp120-expressing virus particles and infected human cells, thus enhancing their uptake and destruction by the human immune system.

(2) By binding the viral glycoprotein gp120, inhibiting its interaction with the human protein CD4 and preventing virus entry.

ARM-H is capable of simultaneously binding gp120, a component of the Env surface viral glycoprotein (found on the surface of both HIV and virus-infected cells) and anti-2, 4-dinitrophenyl antibodies (already present in the human bloodstream). The ternary complex formed between the antibody, ARM-H, and gp120 is immunologically active and leads to the complement-mediated destruction of Env-expressing cells.

9. Blocking of transmembrane signaling ²¹

This is accomplished by blocking the target cell’s polyphosphoinositide pathway and, in turn, the generated second messengers, calcium release and the triggering of protein kinase C (PKC), thus rendering the cells refractory to HIV attack (both gp120-stimulated cell activation and/or triggering of apoptosis). This is achieved by lithium in combination with antiretroviral(s), both requiring obligatory encapsulation in liposomes. FTL/AZT/PEBA, which stands for “freezethawed liposomes/ azidothymidine/ potent enzyme blocking agent”, the latter being liposome-encapsulated lithium ions, are under clinical trial.

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Received on 02.04.2012

Modified on 25.04.2012

Accepted on 04.05.2012

Research J. Pharmacology and Pharmacodynamics. 4(3): May-June, 2012, 158-162