The
antidiabetic biguanide metformin exerts antiproliferative effects in different solid
tumors. However, during preclinical studies,
metformin concentrations required to induce cell growth arrest were invariably within the mM range, thus difficult to translate in a clinical setting. Consequently, the search for more potent
metformin derivatives is a current goal for new drug development. Although several cell-specific intracellular mechanisms contribute to the anti-
tumor activity of
metformin, the inhibition of the
chloride intracellular channel 1 activity (CLIC1) at G1/S transition is a key events in
metformin antiproliferative effect in
glioblastoma stem cells (GSCs). Here we tested several known
biguanide-related drugs for the ability to affect
glioblastoma (but not normal) stem cell viability, and in particular:
phenformin, a withdrawn
antidiabetic drug;
moroxydine, a former
antiviral agent; and
proguanil, an
antimalarial compound, all of them possessing a linear
biguanide structure as
metformin; moreover, we evaluated
cycloguanil, the active form of
proguanil, characterized by a cyclized
biguanide moiety. All these drugs caused a significant impairment of GSC proliferation, invasiveness, and self-renewal reaching IC50 values significantly lower than
metformin, (range 0.054-0.53 mM vs. 9.4 mM of
metformin). All
biguanides inhibited CLIC1-mediated ion current, showing the same potency observed in the antiproliferative effects, with the exception of
proguanil which was ineffective. These effects were specific for GSCs, since no (or little) cytotoxicity was observed in normal umbilical cord mesenchymal stem cells, whose viability was not affected by
metformin and
moroxydine, while
cycloguanil and
phenformin induced toxicity only at much higher concentrations than required to reduce GSC proliferation or invasiveness. Conversely,
proguanil was highly cytotoxic also for normal mesenchymal stem cells. In conclusion, the inhibition of CLIC1 activity represents a
biguanide class-effect to impair GSC viability, invasiveness, and self-renewal, although dissimilarities among different drugs were observed as far as potency, efficacy and selectivity as CLIC1 inhibitors. Being CLIC1 constitutively active in GSCs, this feature is relevant to grant the molecules with high specificity toward GSCs while sparing normal cells. These results could represent the basis for the development of novel
biguanide-structured molecules, characterized by high antitumor efficacy and safe toxicological profile.