Biology Reference
In-Depth Information
that the toxic side effects of NRTIs are correlated with the kinetics of incorporation
by the mitochondrial DNA polymerase (Lee et al.
2003
). Others tested all FDA
approved NRTIs and provided a method to screen NRTIs for potential toxicity.
Furthermore, they defined a toxicity index for chain terminators to account for
relative rates of incorporation versus removal (Johnson et al.
2001
).
Consequences of DNA damage by NRTI incorporation and subsequent chain
termination have been compiled under the following categories: micronuclei, sister
chromatid exchanges, chromosomal aberrations, and telomere shortening (IARC
2000
).
16.3 Zidovudine Becomes Incorporated into DNA
The widespread chronic use of Zidovudine as first line therapy for HIV generated
some concern about patients who may be susceptible to drug-induced genotoxicity.
It was hypothesized that despite the high affinity of Zidovudine for HIV-1 reverse
transcriptase, some incorporation into eukaryotic DNA would be predicted via host
polymerases (Copeland et al.
1992
; Furman et al.
1986
). Numerous laboratories
employing diverse techniques reported to have found Zidovudine incorporated into
DNA of eukaryotic cells, in vivo as well as in vitro. Among other approaches, an
original
32
P postlabeling method to detect Zidovudine-DNA incorporation in mice
(Fang and Beland
2000
), incorporation of Zidovudine in bone marrow using
radiolabeled Zidovudine, and subsequent analysis by high performance liquid
chromatography (Sommadossi et al.
1989
) and incorporation of Zidovudine into
CEM T-lymphoblastoid cells (Avramis et al.
1989
) have been reported. Further-
more, incorporation of Zidovudine into DNA of the chronic myelogenous leuke-
mic cell line K562 and evidence of its removal by an exonucleolytic repair enzyme
(Vazquez-Padua et al.
1990
) was demonstrated in vitro. Removal of ZDV incor-
porated into DNA by a 3'-5' exonuclease able to remove DNA terminated by a
variety of dideoxynucleosides (Skalski et al.
1995
) has also been demonstrated in
leukemic H9 cells (Agarwal and Olivero
1997
). In 1994, Olivero et al. (Olivero
et al.
1994
) used a radioimmunoassay to show Zidovudine-DNA incorporation and
validated the method with radiolabeled compound. Similar levels of Zidovudine-
DNA incorporation were found in mouse. Although variable and depending on
metabolic activation from species to species and from cell type to cell type,
incorporation of Zidovudine into DNA has been extensively documented.
As a consequence of the persistent, non-repaired incorporation, the genotoxic
insult can be perpetuated. Thus, the inability of the cell to effectively remove the
incorporated molecules is translated as mutations and or other forms of genotoxic
events.
