Could cell enzyme activity explain variability in response to NRTIs?

HIV infection produces a wide variation in the activity of two cell enzymes that are crucial for nucleoside analogue (NRTI) activity, according to a test-tube study published in the 25^th March edition of AIDS. This may explain the variability in the effectiveness of these drugs in patients taking antiretroviral therapy.

NRTIs, such as AZT (zidovudine, Retrovir), 3TC (lamivudine, Epivir) and d4T (stavudine, Zerit), are not active against HIV in their native state, but must be converted to the active drug form within human cells. This process, called phosphorylation, is controlled by enzymes produced by the cell.

To assess whether variations in the activity levels of these enzymes are seen in HIV-positive patients, investigators from Italy measured their activity levels in white blood cells taken from 45 HIV-positive patients who were not taking antiretroviral therapy and comparing them to 26 uninfected control patients.

They investigated two enzymes: thymidine kinase (TK) and deoxycytidine kinase (dCK). TK is involved in the conversion of thymidine analogues, such as AZT and d4T. In contrast, dCK controls the formation of the active form of cytidine analogues, such as 3TC.

“Our data indicate strongly that the degree of inter-individual variability in the activity of TK and, to a lesser extent, of dCK in peripheral blood mononuclear cells from antiretroviral-naïve, HIV-infected patients is much greater than that observed in healthy volunteers,” write the researchers. “The data strongly suggest that the activation of these drugs could vary considerably between individuals.”

The investigators saw that there was a broad range in activity levels of TK in the blood cells from the HIV-positive patients when they added AZT to the test tube (0.08 to 2.0U/mg). In contrast, there was much less variability in the HIV-negative control patients (0.08 to 0.3U/mg). The level of activity was also higher in the HIV-positive group (p

The response of the cells to the addition of the enzyme’s natural substrate, thymidine, was also more variable in the HIV-positive patients.

Similarly, activity levels of dCK were more variable in the HIV-positive group (0.17 to 8.4 vs. 3.1 to 5.5U/mg). However, for this enzyme, the activity levels were significantly lower in the HIV-positive group (p

There was no significant correlation between either enzyme’s activity level and CD4 cell count, viral load, age or gender.

“This lack of extensive variability in the activity of these enzymes in healthy volunteers combined with the lack of correlation between TK activity and parameters such as age or sex, may suggest that the variability has no genetic basis, but, rather, depends on HIV infection,” the investigators conclude.

“The determinants of such variability in HIV-infected individuals remain to be established and further studies are needed to explain this phenomenon.”

The researchers speculate that the elevated activity of TK in HIV infection may be caused by the increased cell activation caused by the virus, as TK exhibits its highest activity levels in cells that are dividing. HIV is known to stimulate activity and division of the cells it infects.

In contrast, the cause of the reduction in dCK activity is less clear. “The reason for its decrement and variability in HIV-infected patients is less obvious,” the researchers write. “However, it is possible to speculate that the decreased activity of this enzyme could be caused by the effects of virus replication on cell viability and that different rates of replication may result in different levels of enzyme expression.”

The authors acknowledge that a direct link between variations in enzyme and NRTI activity is not proven by their findings. “It remains to be established whether the observed differences and the variability of expression of these enzymes may affect the rate of dideoxynucleotide formation and, ultimately, whether the above findings have clinical significance,” they write.

“Nevertheless our data indirectly indicate that: Phase I clinical trials, which are usually conducted in healthy volunteers, may not give accurate information on the metabolism in HIV-positive patients of drugs that require the activity of these enzymes.”