Lipidoids: A Combinatorial Approach to siRNA Delivery - RNA Interference from Biology to Therapeutics - page 153

following a single injection [ 49 ]. The new lead material, C12-200, was also used to
deliver siRNA targeting the therapeutically relevant gene transthyretin in NHP and
was shown to be the most potent carrier examined in primates to date [ 49 ] .
Thus, by varying the chemical structure of the final tail, combining existing lipi-
doids in binary pairs to identify synergistic relationships, and leveraging a related
synthetic scheme, the utility of the original lipidoid library was extended. These
studies yielded structure-activity relationships, insights into mechanism of action,
and materials exhibiting enhanced potency.

7.8 Conclusion

Chemical methods were developed to allow the rapid synthesis of large libraries of
structurally diverse lipidoids. The members of the libraries were screened for the
ability to transfect mammalian cells in vitro, and the top-performing materials were
formulated for evaluation in vivo. The lead compound from the first-generation
library, 98N 12 -5(1), was tested in four species, including NHP, and was shown to
facilitate durable, potent, and specific silencing of therapeutically relevant endoge-
nous gene transcripts. Importantly, the knockdown resulted in physiologically rele-
vant outcomes in disease models of hypercholesterolemia, infectious disease, and
cancer. Lipidoids can be administered locally—to the lung or intraperitoneal cavity—
or systemically. The formulation was optimized to maximize uptake by hepatocytes
and was demonstrated to be safe. Notably, inspection of the chemical functional
groups common to the top-performing lipidoids from the first-generation library
informed the synthesis of future-generation libraries, which exhibited a dramatic
increase in the percentage of effective members, including members with greatly
enhanced potency. This combinatorial synthetic approach led to a convergence of
structure and represents an important strategy to expand the scale and diversity of
siRNA delivery reagents. Such expansion enhances the likelihood of identifying
compounds with improved efficacy, resulting in decreased dose as well as increased
tolerability and silencing, facilitating the realization of the potential of RNAi thera-
peutics in the clinic.

7.9 Future Perspectives

The realization of the tremendous potential of RNAi therapeutics depends on the
ability to deliver siRNA successfully into cells in vivo. The single-step synthetic
methodologies employed allow for the simple, parallel generation of large combi-
natorial libraries. The development of lipidoids has expanded the scale and diversity
of available delivery options. These materials confer efficient delivery to liver [ 49 ] ,
lung [ 18 ] , and peritoneal macrophages [ 18 ]. Lipidoids were also used to treat mul-
tiple models of ovarian cancer [ 44, 45 ]. It is likely that consideration of the follow-
ing issues will yet improve the lipidoid platform as well as other delivery systems.