RNAi approaches have been widely combined with
platinum-based
anticancer agents to elucidate cellular responses and to target gene products that mediate acquired resistance. Recent work has demonstrated that platination of
siRNA prior to transfection may negatively influence RNAi efficiency based on the position and sequence of its
guanosine nucleosides. Here, we used detailed spectroscopic characterization to demonstrate rapid formation of Pt-
guanosine adducts within 30 min after coincubation of
oxaliplatin [OxaPt(II)] or
cisplatin [CisPt(II)] with either
guanosine monophosphate or
B-cell lymphoma 2 (BCL-2)
siRNA. After 3 h of exposure to these
platinum(II) agents, >50% of BCL-2
siRNA transcripts were platinated and unable to effectively suppress
mRNA levels.
Platinum(IV) analogues [OxaPt(IV) or CisPt(IV)] did not form Pt-
siRNA adducts but did display decreased in vitro uptake and reduced potency. To overcome these challenges, we utilized biodegradable methoxyl-poly(
ethylene glycol)-block-poly(ε-
caprolactone)-block-poly(
l-lysine) (
mPEG-b-PCL-b-PLL) to generate self-assembled
micelles that covalently conjugated OxaPt(IV) and/or electrostatically complexed
siRNA. We then compared multiple strategies by which to combine BCL-2
siRNA with either OxaPt(II) or OxaPt(IV). Overall, we determined that the concentrations of
siRNA (nM) and
platinum(II)-based
anticancer agents (μM) that are typically used for in vitro experiments led to rapid Pt-
siRNA adduct formation and ineffective RNAi. Coincorporation of BCL-2
siRNA and
platinum(IV) analogues in a single
micelle enabled maximal suppression of BCL-2
mRNA levels (to <10% of baseline), augmented the intracellular levels of
platinum (by ∼4×) and the numbers of resultant Pt-
DNA adducts (by >5×), increased the cellular fractions that underwent apoptosis (by ∼4×), and enhanced the in vitro antiproliferative activity of the corresponding
platinum(II) agent (by 10-100×, depending on the
cancer cell line). When combining RNAi and
platinum-based
anticancer agents, this generalizable strategy may be adopted to maximize synergy during screening or for therapeutic delivery.