Viral fitness

The influenza virus inhibitors, zanamivir, and oseltamivir, act outside the cell after virus particles have been formed. The dtugs have been designed to fit into the active site of the viral envelope enzyme neuraminidase, which is required to cleave sialic acid off the surface of the producing cells. When its activity is blocked, new virus particles stay attached to the cell surface through binding of the virus protein hemagglutinin to sialic acid and are prevented from spreading to other cells. 

Toxicology studies must be performed in at least two animal species. If the toxicity profile of the compound is acceptable, then it joins the hit or lead list of compounds to proceed. The metabolism of the compound must be understood and pharmacokinetic studies must be performed in small and large animals. Efficacy studies must be performed in relevant animal models, especially in chimpanzees when more than one candidate is identified and a choice has to be made before proceeding to studies in humans. The ultimate preclinical steps include various studies testing drug combinations in vitro and in vivo, selection of resistant viruses, viral fitness, pyrophosphorolysis, and others. 

HCV polymerase that carried the S282T mutation did no longer incorporate 2 -C-methyl-CTP during the initiation step of RNA synthesis (Dutartre et al. 2006). The presence of the S282T mutation induces a general reduction (5-20-fold) in terms of polymerase efficiency (Dutartre et al. 2006), which may translate to decreased viral fitness (Ludmerer et al. 2005). 

Strain Mutation Drug Level of sialidase activity and/or viral fitness... 

Our understanding of evolution and the role of viral diversity, resistance, and fitness has expanded greatly over the last decade, but is still incomplete. Undoubtedly, our insights will improve in the years to come. 

As for HIV, the selection of HCV drug resistant variants can be accompanied by a decrease in RC. For HCV protease inhibitor resistant variants, the level of resistance seems to be inversely related to viral fitness (Sarrazin et al. 2007 Tong et al. 2006 Yi et al. 2006). There is some evidence that viral RC can be restored by the selection of compensatory mutations within the protease gene (Sarrazin et al. 2007 Tong et al. 2006 Yi et al. 2006). However, further research is warranted to investigate to what extent viral fitness can be restored and by which mutations. Also for the nucleoside and non-nucleoside inhibitors, the selection of resistance results in a fitness defect. It remains to be investigated whether or not compensatory mutations can be selected. 

Changes in human immunodeficiency vims type 1 Gag at positions 1449 and P453 are linked to I50V protease mutants in vivo and cause reduction of sensitivity to amprenavir and improved viral fitness in vitro. J Virol 76 7398-7406... 

Mammano F, Petit C, Clavel F (1998) Resistance-associated loss of viral fitness in human immunodeficiency vims type 1 phenotypic analysis of protease and gag coevolution in protease inhibitor-treated patients. J Virol 72 7632-7637... 

In the clinically relevant concentration range56 the relation between the response of the sensor and the viral concentration is linear (a linear fit through the data points in Fig. 10.16a gives a correlation coefficient of 0.98) facilitating easy virus concentration predictions with a calibrated sensor. Furthermore, even at the lowest measured virus concentration (850 particles/ml) a high signal-to-noise ratio of... 

Limitations on the size of the insert. In some cases the needed DNA sequence is too large to fit in the viral vectors. 

Initiation of reverse transcription in HIV-infected cells relies on a critical RNA-RNA interaction between tRNA y s, which is preferentially packaged into the viral particle, and a specific viral RNA seqnence. The 3 -terminaI 18 nucleotides of tRNA y are complementary to the primer binding site (PBS) sequence located in the 5 -Iong terminal repeat (LTR) of the viral RNA genome (Figure 10.3). The UUU anticodon of the tRNA is complementary to and binds to an adenosine rich loop located 8 nucleotides upstream (5 ) of the PBS. This RNA-RNA duplex which is formed when tRNA y s binds to the PBS fits within the active site of HIV-1 reverse transcriptase, bnt mnitiple interactions between the viral RNA and tRNA y are necessary for efficient initiation of reverse transcription. This interaction nucleates the reverse transcription complex which contains viral RNA, reverse transcriptase, tRNA y pl , nncleocapsid p7, and Vpr (Viral protein R), as well as multiple host factors." ... 

TC can be considered to be maintained in second-Une regimens to potentially reduce viral fitness, confer residual antiviral activity and maintain pressure on the Ml 84V mutation to improve viral sensitivity to AZT or TDF. AZT may prevent or delay the emergence of the K65R mutation. 

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