Chemistry Reference
In-Depth Information
The final steps included formation of the double bond by oxidation and
elimination of iodine, conversion of the purine methoxy group to an amino
group, and dealkylation of the phosphonate esters to provide target 22 over
eight steps with an overall yield of
10%. Multigram quantities were prepared
using this route, enabling prodrug development and 5-day toxicology.
B
10.3 Amidate Prodrug Development to Identify
GS-9131
10.3.1 Designing Prodrugs that Target Lymphoid Cells
The disoproxil prodrug 2 effectively improved the cell permeability properties
of 1 for oral delivery. However, the ideal prodrug not only improves cell perm-
eability but also survives hepatic and plasma enzymes to enable sucient
plasma levels of prodrug to diffuse into lymphoid cells and then be pre-
ferentially cleaved within the lymphoid cells. Monoamidate 11 was developed
with these goals in mind and succeeded in improving the delivery of 1 to
PBMCs compared to prodrug 2, both in beagle dogs and initial proof-of-
concept clinical trials.
9,10
The intracellular cleavage enzyme inside PBMCs was
later discovered to be lysosomal cathepsin A (Cat A), a protease highly
expressed in PBMCs.
31
The proposed breakdown of the prodrug begins with
Cat A-mediated cleavage of the amino acid ester, followed by intramolecular
cyclization to 28, and ejection of phenol as shown in Figure 10.7.
Hydrolysis of the P-O bond of cyclic intermediate 28 followed by chemical or
enzymatic cleavage of the P-N bond then releases 1 into the cytosol. The
strategy adopted for 22 was to design a range of amidate prodrugs similar to 11,
and profile the prodrugs for cleavage by recombinant Cat A, antiviral activity,
and stability toward hepatic, plasma, and intestinal enzymes as shown in
Table 10.4.
25
Amidate 11 provided a useful guidepost for prodrug optimiza-
tion, since it was anticipated that the best prodrugs of 22 would equal or exceed
the stability properties of 11.
Initially, a range of bisamidate prodrugs bearing natural and unnatural
amino acids were explored to avoid the generation of a phosphorus chiral
NH
2
NH
2
i. Cathepsin A
iii. hydrolyse
N
N
ii. cyclise
N
N
O
O
O
N
O
N
N
N
O
O
P
O
P
O
1
H
NH
O
iv. cleave
28
11
Figure 10.7
Proposed four step (i-iv) breakdown pathway for amidate prodrugs.
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