RECENT PROGRESS AND PROSPECTS ON PLANT-DERIVED ANTI-HIV AGENTS AND ANALOGS - Medicinal Chemistry of Bioactive Natural Products - page 389

lysates. Sequence analysis of DSB-resistant mutants indicated that a single muta-
tion encoding an amino acid change (A to V) at the N terminus of SP1 was found.
After introduction of this mutation A-to-V into the NL4-3 backbone to generate the
NL4-3 SP1/A1V mutant, DSB had no effect on the processing of the mutant Gag,
as no p25 was detected at 1 mg/mL. These results indicate that the A-to-V substitu-
tion at the p25 cleavage site confers DSB resistance. 92
A similar study by Zhou et al. 93 indicates that a single mutation, L363F, at the P1
position of the CA-SP1 junction confers resistance to DSB. Additional evidence for
the Gag protein as the target of DSB was obtained through substitution of amino
acid residues flanking the CA-SP1 junction of HIV-1 or the simian immuodefi-
ciency virus (SIV). This study takes advantage of the fact that DSB does not affect
the replication of SIV. Alteration of three residues in the SIV CA-SP1 junction to
the corresponding amino acids of HIV-1 resulted in an SIV mutant, with a similar
degree of sensitivity as HIV-1 to DSB. On the other hand, two substitutions in HIV-1
with the corresponding residues of SIV resulted in resistance to DSB. These results
suggest that the CA-SP1 junction is a key determinant of DSB sensitivity. 94 There-
fore, DSB represents a unique class of anti-HIV compounds termed maturation
inhibitors (MIs) that exploit a previously unidentified viral target, which provides
additional opportunities for HIV drug discovery.
9.4.3 Bifunctional Betulinic Acid Derivatives with Dual
Mechanisms of Action
The viral targets of BA derivatives vary, depending primarily on the side-chain
structures of the compounds. 62 RPR103611 (246) and IC9564 (257) are entry inhi-
bitors, and DSB (263) targets viral maturation. Therefore, the BA skeleton probably
serves as a scaffold and binds to a common motif. Based on the molecular targets
and chemical structures of IC9594 and DSB, Huang et al. reported on the synthesis,
bioevaluation, and mechanism of action study of two new compounds, LH15 (282)
and LH55 (283), which have a 3 0 ,3 0 -dimethylsuccinyl ester at C-3, similar to DSB.
LH-15 (282) is a leucine amide, and LH-55 (283) is an 11-aminoundecanoic amide
derivative. 95 Their activities against several HIV strains are listed in Table 9-12.
LH-55 (283) showed the best activities; its EC 50 values ranged from 0.03 to
0.006 mM against a wild-type strain (NL4-3), and two drug-resistant strains, PI-R
and RTI-R. LH-15 (282) was two-to five-fold less potent than 283, but more potent
than the maturation inhibitor DSB (263) and the entry inhibitor IC9564 (257)
screened side by side. 95
The side chain at position-28 is critical for the antifusion activity of the BA deri-
vatives, whereas a dimethylsuccinyl acid at position-3, as in DSB (263), results in
inhibition of viral maturation rather than affecting the envelope-mediated mem-
brane fusion. The key structural feature that enables LH55 (283) and its analogs
to possess a dual mode of action is the presence of side chains at both C-3 and
C-28. The unique biological activity of these compounds is that they not only inhi-
bit viruses that are resistant to HIV-1 RT and PR inhibitors, but also they inhibit
viruses that are resistant to compounds with side chains only at position-3 or