Biomedical Engineering Reference
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particle. PEG-lipid length was varied between 10 and 16 carbons. At
2.5 mg/kg, mouse weight loss remained constant as lipid length
changed, but differed dramatically at 25 mg/kg. At 25 mg/kg, lipids
with 14, 15, or 16 carbons were tolerated better than chains between
10 and 13 carbons. At the same time, efficacy was maximized for
tails 13 to 15 carbons in length. These studies demonstrated that
alkyl tails with 14 or 15 carbons were optimal. Since even-numbered
carbon tails are less expensive, C14 was chosen for subsequent
studies, which sought to correlate particle size and efficacy.
To control particle size, 98N
-5 particles were made with
identical molar ratios and extruded through membranes with
different pore sizes. This produced particles with diameters of
150, 85, 60, and 50 nm, respectively. F7 levels measured 48 h after
intravenous injection at a siRNA dose of 3 mg/kg clearly showed
improved silencing as size decreased. These results were expected,
since smaller particles enter into liver parenchyma through
fenestrated hepatic vasculature more easily. Liver distribution
was later verified using radioactively labeled 98N
12
-5. Following
intravenous injection, particles were observed predominantly in the
liver (92% of injected dose), with a small percentage found in the
spleen, another fenestrated organ.
12
7.6
Synergy: Combining Existing Compounds to
Achieve Improved Delivery
The iterative library approach identified 98N
-5 as the most potent
molecule from an original library of over 1,200 compounds. A large
percentage from the original library was ineffective, presumably
because delivery requires both that the vehicles enter the endosome
and escape it to enter the cytoplasm. Members of our lab reasoned
that some lipidoids might promote endosomal uptake without
affecting escape, and vice versa. They investigated whether combining
two ineffective lipidoids would produce a synergistic effect on
delivery.
12
They envisioned that one lipidoid would facilitate uptake
while the second would foster escape.
To test this idea, 36 lipidoids were combined to form 630 binary
pairs. The mass ratio of each lipidoid pair was varied between
zero and one with a step size of 0.2, resulting in nearly 4,000
formulations.
19
19
Since the purpose of the library was to investigate the
effects of binary lipidoid combinations,
in vitro
knockdown in dual
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