Biomedical Engineering Reference
In-Depth Information
The discovery of VU0184670 and VU035017 has provided highly subtype-
selective agonists for the M
1
acetylcholine receptor. The compounds discovered
supplement known allosteric M
1
modulators. Moreover, the compounds' clean ancil-
lary pharmacology contrasts with the off-target activity observed for M
1
agonists
described previously and for the benchmark allosteric M
1
agonist TBPB [77]. With a
well-characterized binding mechanisms and demonstrated efficacy in animal models,
these compounds may prove to be useful probes for expanding biological studies
of the acetylcholine muscarinic M
1
receptor and for developing new treatments for
cholinergic-related cognitive disorders.
18.4.3
Inhibitors of Protein Prenylation: Probes with Novel Structures
Inhibitors of protein prenylation have received much attention as potential anticancer
agents due to a number of oncogenic proteins (e.g., Ras proteins) that undergo a
posttranslational farnesylation or geranylgeranylation that facilitates their anchoring
to the plasma membrane [78]. It is known that when FTase-catalyzed farnesylation
of the Ras isoform K-RasB is blocked, GGTase-catalyzed geranylgeranylation of the
protein can substitute functionally for farnesylation [79]. Consequently, inhibitors
of both farnesyltransferase (FTase) and geranylgeranyltransferase (GGTase) may be
necessary to fully block prenylation of oncogenic Ras proteins. Whereas extensive
efforts have been made to develop FTase inhibitors, there have been fewer studied
aimed at identifying inhibitors for GGTase. Two varieties of GGTase have been
characterized. The first, GGTase-I, catalyzes the geranylgeranylation of Rho proteins
of the Ras superfamily and heterotrimeric G-proteins at the cysteins of C-terminal
CAAX sequences (A is an aliphatic amino acid, and X is leucine or phenylalanine)
[80]. The second, RabGGTase, catalyzes the geranylgeranylation of two cysteins on
the C-termini of the secretory and endocytosis-related Rab proteins, members of the
Ras superfamily, at CC or CXC sequences [81].
Kwon and co-workers have developed a new class of geranylgeranylation
inhibitors by screening aDOS library of 4288 compounds against GGTase-I-catalyzed
geranylgeranylation of the two Ras superfamily proteins RhoA and K-Ras4B [38].
In the screen, the incorporation of tritiated (
3
H) geranylgeranyl groups into the Ras
proteins was measured. The compounds P5-H6 and P3-E5, the two most active hits
(Figure 18.5), were used in a follow-up study with HEK-293 cells transfected with
the Ras oncogene Rheb-CSVL. Both compounds were found to markedly decrease
the levels of prenylated Rheb-CSVL. Furthermore, these two compounds exhibited
a significant specificity for GGTase-I over both RabGGTase and FTase.
Optimizations of P5-H6 led to a more potent compound, P61-A6, which showed
a 10-fold increased antiproliferative activity in K562 cells and which appreciably
inhibited geranylgeranylation of Rap1, another member of the Ras superfamily that
undergoes GGTase-I-catalyzed geranylgeranylation, at 2.5
M [38b]. Follow-up
studies suggested that the greater cellular activity of P61-A6 may have resulted from
improved stability or absorption rather than inherent affinity for GGTase-I [82]. When
P61-A6 was used as a GGTase-I inhibitor to treat xenograft pancreatic (PANC-1)
