The phenomenon of multidrug resistance (MDR) in cancer cells is generally associated
with P-glycoprotein (P-gp) expression and presents an obstacle to successful
chemotherapy. Attempts to overcome P-gp-associated MDR using P-gp modulators, such
as verapamil, have been hindered by their intrinsic in vivo toxicity. In 1991, however, Scala
et al. demonstrated the alteration of P-gp function by interferon-alpha (IFN-α) in vitro at
non-toxic in vivo concentrations, suggesting a basis for the use of IFN-α clinically in
patients exhibiting P-gp-associated MDR.
Drug resistance in B-CLL has been linked to the phenomenon of MDR, however,
publications regarding this have been conflicting. The contrasting results prompted further
investigation of the role of P-gp-associated anthracycline resistance and, using isolated β-lymphocytes
from B-CLL patients, this investigation examined P-gp expression, function
and IFN-α modulation in vitro.
Optimum conditions for in vitro analysis of P-gp-associated anthracycline resistance were
determined by examining the stability of the anthracycline, daunorubicin, in varying cell
culture conditions. The resulting system balanced conditions affecting drug stability with
those affecting cell survival. While other investigations have neglected the issue of drug
stability, this study demonstrates that the instability of daunorubicin may be a critical
variable determining the outcome of drug sensitivity studies. In RPMI + 2mM L-glutamine
and 10% (v/v) FBS, loss of drug concentration is due to both adsorption and degradation
and these experiments show that the presumed availability of drug may be over-estimated
in in vitro studies. Furthermore, the degradation products might interfere with P-gp
function and modulation.
MDRl gene mRNA was detected in the B-cells of forty-three out of fifty B-CLL patients
analysed, whereas P-gp expression, as measured by flow cytometry, resulted in only sixteen
patients out of fifty-five being classed as positive (> 10% increase in staining as compared
to the control). P-gp functionality and modulation studies on the B-cells of eleven patients
confirmed the existence of an efflux mechanism with identical characteristics to P-gp using
verapamil, the dye rhodamine 123 (rho123) and daunorubicin. Four patients were classed
as functional low expressers (functional P-gp with low P-gp expression (7-10% increase in
staining)), six were classed as functional high expressors (functional P-gp with high P-gp
expression (20-57% increase in staining)) and one as a non-functional high expressor (non-functional
P-gp with high P-gp expression (13.4% increase in staining)). Verapamil
modulated rho123 efflux in all ten patients classed as P-gp functional expressors, and
daunorubicin efflux in eight of these patients. However, IFN-α modulated rho123 and
daunorubicin efflux in only two and one patients, respectively, even at concentrations
higher than 500I.U./ml. In contrast to Scala et al. (1991), this finding suggests that at a well
tolerated concentration IFN-α may not be suitable for use as a P-gp modulating agent in
vivo in B-CLL, although conclusive evidence would require a larger study.
Date of Award | 2001 |
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Original language | English |
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Awarding Institution | |
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P-GLYCOPROTEIN-ASSOCIATED ANTHRACYCLINE RESISTANCE IN B-CLL: P0TENTIAL FOR CYTOKINE MODULATION
MUNOZ-RITCHIE, V. G. (Author). 2001
Student thesis: PhD