STRUCTURE MODIFICATIONS AND THEIR INFLUENCES ON ANTITUMOR AND OTHER RELATED ACTIVITIES OF TAXOL AND ITS ANALOGS - Medicinal Chemistry of Bioactive Natural Products - page 116

seventh to eighth TMS region. Because the tertiary structure of MDR1 or mdr1b is
still unknown, the above results may help to identify the binding domain of hydro-
phobic molecules on the P-gP, which thus leads to the rationale of the MDR
mechanism and the structure-based design of new P-gP inhibitors.
Besides P-gP overexpression, alteration of tubulin isotypes, amino acid muta-
tions in tubulin, and microtubule dynamic changes are also involved in resistance
to paclitaxel and other anti-microtubule agents. Here we concentrate on some recent
results. Comprehensive descriptions can be found in reviews by Burkhart et al. 197
and Sangrajrang et al. 198
Drug-induced alteration of tubulin isotype expression in resistant cells is consid-
ered to be a general mechanism of antimitotic drugs resistance. 198 Research on the
distribution of different tubulin subtypes showed that in MDR tumor cells, b-II and
b-IVa isotype tubulins were absent, whereas the level of b-III tubulin increased. 199
Direct evidence for the involvement of such alteration is that antisense oligonucleo-
tides to class III b-tubulin isotype did enhance sentivity about 30% to paclitaxel in
resistant lung cancer cells. 200 These results indicated that b-III tubulin isotype is a
biomarker of resistance. Other cellular factors binding to tubulin, e.g., microtubule
associating protein 4 (MAP4), 201 rather than tubulin itself, are also responsible for

the resistance.
Amino acid mutations also make contributions. Gonzalez-Garay et al. identified
a cluster of mutations in class I b-tubulin isotype. 202 All six mutants had substitu-
tions at leucines, such as Leu215, Leu217, and Leu228, which may lead to desta-
blization of a microtubule. This finding also claimed the importance of the leucine
cluster in microtubule assembly. Giannakakou et al. found some b-tubulin muta-
tions in paclitaxel and epothilone-resistant tumor cells. These mutations are
Phe270 to Val, Ala364 to Thr, Thr274 to Ile, and Arg282 to Gln. The first two muta-
tions cause paclitaxel resistance, whereas the latter two cause resistance to both
paclitaxel and epothilone. 203 In a recent report, two mutants were isolated from
tumor cells resistant to both paclitaxel and desoxyepothilone B. The mutant of
Ala231 to Thr locates at helix-7 within the binding pocket of paclitaxel derived
from crystallographic data, whereas Gln292 to Glu in helix-9 near the M loop, out-
side of the taxol binding site. 201 Recently, a clinical report demenostrated the cor-
relation between tubulin mutant and paclitaxel resistance in non-small-cell lung
cancer patients. The group of patients without mutations had longer median survi-
val time and higher 1-, 3-, and 5-year survival rates. 204 Also, a-tubulin mutations
are widely distributed in paclitaxel-resistant cells, and the evidence for their roles
to confer resistance is still indirect. More studies are needed to figure out whether
a- and b-tubulin are involved in resistance in a synergic way.
Microtubule dynamics alterations also showed impacts on resistance. Goncalves
et al. found a 57% increase of microtubule instability in paclitaxel-resistant cell
A549-T12 as compared with parental sensitive cell A549, and a 167% overall
increase in the more resistant cell A549-T24. 205 It is interesting to note that the
resistant cells, in the absence of paclitaxel, suffered mitotic block as well, which
suggests that both increased or suppressed microtubule dynamics can impair cell
function and proliferation.