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bound to PVA–SPTX. On the other hand, the stability of PTX as forming a polymeric lipid nanosphere.38) As shown
PTX is expressed by the following equation:
in Fig. 6, PVA–SPTX inhibited the proliferation of L1210
cells to the same extent as PTX dissolved in the Purebright
MB37-50T. The cytotoxicity of PTX in the Purebright
[PTX]ꢂ[PTX]0 exp(ꢄk2t)
(2)
where [PTX]0 is the initial concentration of PTX. At first the MB37-50T was similar to that reported by Wada et al.39)
parameters of k2 were calculated from the observed time
In the previous work, we examined the pharmacokinetics
courses shown in Fig. 3 using Eq. 2. Equation 1 was then and biodisposition of PVA in experimental animals. [125I] la-
fitted to the observed time courses shown in Fig. 4 using a beled PVA was retained in the blood circulation for several
nonlinear least squares program (MULTI).35)
days after intravenous injection to mice. Although the tissue
The model suggested in Chart 1 suffered the divergence in distribution of PVA was small, a significant accumulation in
most of the computations; even if it was converged, the nega- the liver, kidney and spleen was observed.27) In this study,
tive value of k3 with an extraordinarily large standard devia-
[
125I]-PVA–SPTX was also retained in the blood circulation
tion was obtained. When it was assumed that the degrada- very well and was gradually accumulated in the tumorous tis-
tionof PTX bound to PVA is negligible, however, the excel- sue as shown in Fig. 7. These findings indicated that PVA–
lent convergence could be obtained in the curve fitting using SPTX was accumulated efficiently in the tumorous tissue by
the algorithm of Gauss–Newton method where the following EPR effect.22—24)
equation was adopted instead of Eq. 1.
In conclusion, in this study the conjugate was generated
from the combination of PVA and PTX for the first time. It
was suggested that the water-solubility of PTX was markedly
[PTX]ꢂk1[PTX]*/(k2ꢄk1)ꢃ(exp(ꢄk1t)ꢄexp(ꢄk2t))
(3)
The apparent first-order rate constants for regeneration of enhanced by the conjugation to PVA, and the conjugate,
PTX from PVA–SPTX (k1) and for decomposition of PTX PVA–SPTX, effectively delivered PTX to the tumorous tis-
(k2) are depicted in Fig. 5. The profiles of k1 and k2 showed sue due to the EPR effect. These studies demonstrated that
pH-dependency in the investigated pH range. PTX was very PVA may be used as an effective solubilizing carrier for
stable at acidic conditions, but the decomposition was in- PTX.
creased with the elevation of the pH. It was reported that
PTX was susceptible to mild basic hydrolysis which resulted
Acknowledgements This work was supported in part by
in the formation of baccatin III as the major product.36) The Grants in Aid (No. 13672406) for Scientific Research (C)
regeneration rate of PTX was also enhanced with the rise in from the Ministry of Education, Culture, Sports, Science and
pH and reached a maximum at pH 9. At pH 7, the regenera- Technology, Japan (to YK). The authors are also grateful to
tion of PTX (k1ꢂ0.187 hꢄ1) was 54-times faster than the Mr. Hiroshi Noguchi of Japan Vam & Poval Co., Ltd., Osaka,
degradation of PTX (k2ꢂ0.00345 hꢄ1), showing half-lives of Japan for the gift of the PVA samples. The authors particu-
3.7 and 201 h, respectively. These findings indicate that cova- larly acknowledge T. Sunagari and D. Ishii for their technical
lent binding to PVA stabilizes PTX (k3ꢂ0) and the macro- assistance.
molecular prodrug, PVA–SPTX, releases PTX gradually at
physiological conditions.
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was similar to that measured by the UV method. These find-
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