Lack of reduction of thymidine kinase activity in stavudine-treated HIV-infected patients

It has been demonstrated that prolonged in vivo or in vitro treatment with some nucleosides analogs may favor the selection of cells with a reduced activity of enzymes involved in the phosphorylation of these drugs leading to a reduced sensitivity to their antiretroviral action. The aim of this study was to evaluate the effect, in vivo, of zidovudine and stavudine treatment on thymidine kinase (TK) activity. The results obtained showed that TK levels in PBMC from naive patients and stavudine-treated patients did not significantly differ ( naive TK = 4.16 +/- 1.19 U/mg protein; stavudine TK = 3.65 +/- 1.73 U/mg protein; p = 0.42), suggesting that the treatment with this nucleoside analog is not associated to a defect of TK activity. On the contrary, PBMC from zidovudine-treated patients showed a significant reduction in TK activity compared to naive patients (naive TK = 4.16 +/- 1.19 U/mg protein; zidovudine TK = 2.70 +/- 1.54; p = 0.014. Although the clinical significance of these results has to be established, we can speculate that stavudine and zidovudine, which are presumably phosphorylated by the same cellular kinases, might display a different ability to in vivo select cells with a resistant phenotype

Selection of a T-cell line resistant to stavudine and zidovudine by prolonged treatment with stavudine

It has been demonstrated that prolonged treatment with nucleoside analogues, such as T-azido-T-deoxythymidine (zidovudine), 2',3'-dideoxycytidine (zalcitabine) and 9-(2-phosphonylmethoxyethyl) adenine (PMEA), may cause selection of cells that are resistant to their anti-HIV activity. A human T-lymphoblastoid cell line that is resistant to the antiviral and cytotoxic activity of 2',3'-didehydro-3'-deoxythymidine (stavudine) has developed as a result of prolonged treatment. These cells, called CEMstavudine, are also less sensitive to zidovudine. The cellular/pharmacological resistance acquired by the CEMstavudine cells is relatively low and appears to correlate with a reduction in thymidine kinase (TK) activity, rather than with a decreased expression of TK mRNA

Alteration of thymidine kinase activity in cells treated with an antiviral agent

A lymphoblastoid cell line, CEM, was rendered resistant to zidovudine (AZT) in vitro by exposure to low but gradually increasing concentrations of the drug. This type of cellular resistance seems to be due to a defect of thymidine kinase (TK) activity that is acquired by cells grown in the presence of AZT: In fact, enzymatic studies with extracts from AZT-resistant cells (CEMazt), have shown that the value of the maximum velocity (Vmax) of TK activity measured with AZT and for deoxythymidine (dThd) is decreased as compared to sensitive CEM cells. Further-move, the enzyme affinity for AZT and dThd is reduced in CEMazt. Further experiments have shown that such cells do not show resistance to other nucleoside analogs, such as ddI, ddC: AraT and D4T, suggesting that the phosphorylation pathways different fr om those involving TK are unaltered. Ex vivo experiments performed by using peripheral blood mononuclear cells (PBMC) from HN infected individuals revealed that a prolonged treatment with AZT may modify the affinity of TK for dThd thus suggesting that the aforementioned phenomenon may occur also in vivo

Impaired 2’-3’-dideoxy-3’-thiacytidine accumulation in CEM lymphoblastoid cells as a mechanism of acquired resistance independent of MRP4 with a possible role for ABCC11.

We have developed a human T lymphoblastoid cell line (CEM3TC) that is selectively resistant to the anti-proliferative effect of 2',3'-dideoxy-3'-thiacytidine (3TC) because the CEM3TC cells were equally sensitive to AZT, as well as the antimitotic agent, vinblastine. The anti-retroviral activity of 3TC against HIV-1 was also severely impaired in the CEM3TC cells. Despite similar deoxycytidine kinase activity and unchanged uptake of nucleosides such as AZT and 2'-deoxycytidine, CEM3TC had profoundly impaired 3TC accumulation. Further studies indicated that CEM3TC, retained much less 3TC. However, despite a small overexpression of multidrug resistance protein (MRP) 4, additional studies with cells specifically engineered to overexpress MRP4 demonstrated there was no impact on either 3TC accumulation or efflux. Finally, an increased expression of the MRP5 homologue, ATP-binding cassette C11 (ABCC11) was observed in the CEM3TC Cells' We speculate that the decreased 3TC accumulation in the CEM3TC might be due to the upregulation of ABCC11

Impaired 2',3'-dideoxy-3'-thiacytidine accumulation in T-lymphoblastoid cells as a mechanism of acquired resistance independent of multidrug resistant protein 4 with a possible role for ATP-binding cassette C11.

Cellular factors may contribute to the decreased efficacy of chemotherapy in HIV infection. Indeed, prolonged treatment with nucleoside analogues, such as azidothymidine (AZT), 2',3'-deoxycytidine or 9-(2-phosphonylmethoxyethyl)adenine, induces cellular resistance. We have developed a human T lymphoblastoid cell line (CEM 3TC) that is selectively resistant to the antiproliferative effect of 2',3'-dideoxy-3'-thiacytidine (3TC) because the CEM 3TC cells were equally sensitive to AZT, as well as the antimitotic agent, vinblastine. The anti-retroviral activity of 3TC against HIV-1 was also severely impaired in the CEM 3TC cells. Despite similar deoxycytidine kinase activity and unchanged uptake of nucleosides such as AZT and 2'-deoxycytidine, CEM 3TC had profoundly impaired 3TC accumulation. Further studies indicated that CEM 3TC retained much less 3TC. However, despite a small overexpression of multidrug resistance protein (MRP) 4, additional studies with cells specifically engineered to overexpress MRP4 demonstrated there was no impact on either 3TC accumulation or efflux. Finally, an increased expression of the MRP5 homologue, ATP-binding cassette C11 (ABCC11) was observed in the CEM 3TC cells. We speculate that the decreased 3TC accumulation in the CEM 3TC might be due to the upregulation of ABCC11

P-glycoprotein expression affects the intracellular concentration and antiviral activity of the protease inhibitor saquinavir in a T cell line

A number of ATP-binding cassette proteins, which function as cellular efflux pumps, are known to be expressed on the membranes of human cells, including CD4-positive lymphocytes. It has also been shown recently that most anti-HIV protease inhibitors (PIs) are first-rate substrates of one of these membrane transporters, P-glycoprotein (Pgp). These findings raise the question of whether pgp expression could influence HIV replication and/or affect the action of PIs. To gain new insight into this, initially unexpected, phenomenon, a study was undertaken with the aims of investigating whether treatment with saquinavir (SQV) induces Pgp expression in primary or transformed human T cell lines and, primarily, establishing whether Pgp expression could modify both the uptake of SQV and its antiviral action. Pgp expression, mainly measured by reverse transcription-PCR, was found to be variably detectable in healthy individuals, and short or prolonged SQV treatment was unable to induce or increase the expression of Fgp in a lymphoblastoid cell line or in primary lymphocytes derived from these individuals. However, further experiments, performed in a cell line with high Pgp expression (CEMVBL100 cells) and its parental cell line (CEM cells), demonstrated that over-expression of Pgp reduces the uptake of SQV This result is consistent with the finding that CEM(VBL100)cells are less sensitive to the antiviral activity of SQV the ID50 value (100 muM) being significantly higher than the corresponding value in parental CEM cells (4 muM)