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Inducible and constitutive HSP27, HSP70, and HSP90 mRNA and protein levels were measured by real-time reverse transcription-PCR and immunoblot assays. Compared with no treatment, Act D alone decreased HSP mRNA levels in MM.1S and RPMI-8226 cell lines. Combining Act D with 17-AAG did not attenuate 17-AAG-mediated increases in transcript levels of inducible HSP70; however, constitutive HSP mRNA levels were decreased. In contrast to its effect on mRNA levels, Act D was able to abrogate 17-AAG-mediated increases in all HSP protein levels. The cytotoxicity of combined Act D and 17-AAG was assessed. Treatment with Act D alone caused http://www.selleckchem.com/products/pexidartinib-plx3397.html increase in cell death in both multiple myeloma cell lines (122). IPI-504, the highly soluble hydroquinone hydrochloride derivative of 17-AAG, was synthesized as an HSP90 inhibitor with favorable pharmaceutical properties. A phase I study has been performed in relapsed myeloma patients. IPI-504 was administered over 30?min on day 1, 4, 8 and 11 every 21?d with dose ranging from 90 to 300?mg/m2. No dose-limiting toxicities have been http://www.selleckchem.com/products/bmn-673.html observed (114). The antitumor activity of IPI-504 was tested as both a single agent as well as in combination with bortezomib in myeloma cell lines and in vivo xenograft models, and the retention of IPI-504 in tumor tissue was determined. IPI-504 seems to be efficacious in cellular http://www.selleck.cn/products/nlg919.html and animal models of myeloma. It is synergistically efficacious with the proteasome inhibitor bortezomib and is preferentially retained in tumor tissues relative to plasma. Importantly, it was observed that IPI-504 interconverts with the known agent 17-AAG in vitro and in vivo via an oxidation-reduction equilibrium, and it was demonstrated that IPI-504 is the slightly more potent inhibitor of HSP90 (80). The HSP90 inhibitor 17-DMAG showed significant activity against MM cells both in vitro and in vivo (123, 124). Huston et?al. showed in fact that the combination of the Akt inhibitor perifosine and the HSP90 inhibitor 17-DMAG synergistically induces apoptosis and inhibits MM cell growth and proliferation. Interestingly, the sequence of the two agents indicated that the use of 17-DMAG before perifosine or the use of both agents simultaneously induces higher cytotoxicity compared with using perifosine before the addition of 17-DMAG, suggesting that 17-DMAG sensitizes MM cells to the perifosine-induced cytotoxicity. Perifosine and 17-DMAG inhibited Akt activity and downstream signaling pathways, including phosphorylated S6 kinase. In addition, the combination inhibited HSP90 signaling, including activation of HSP70 and inhibition of HSP27 phosphorylation. Interestingly, 17-DMAG inhibited the ERK MAPK and p38 MAPK pathways, whereas perifosine induced these cascades, suggesting that 17-DMAG may overcome the resistance conferred by ERK activation in MM.