Biomedical Engineering Reference
In-Depth Information
anti-AIDS treatment armamentarium. Most of these NRTIs are nucleoside analogs with the excep-
tion of tenofovir disoproxil fumarate (TDF), which is a nucleotide analog of adenosine phosphate.
The NRTIs are administered as unphosphorylated prodrugs. Upon entering the host cell, these
prodrugs are recognized by cellular kinases and further converted to the tri-phosphorylated form.
The tri-phosphorylated NRTIs then bind to the active site of RT and are catalytically incorporated
into the growing DNA chain. The incorporated NRTIs block the further extension of the chain since
the NRTIs lack the 3
′
hydroxyl group on their ribose or pseudoribose moiety and thus cannot form
the 3
phosphodiester bond needed for DNA extension. NRTIs are one of the major classes of
inhibitors used in all combination therapies for the treatment of HIV-infected patients. However,
the clinical successes of these agents are limited by viral resistances to NRTIs, arising through
mutations in the coding region of RT. These mutations confer viral resistances through improved
discrimination of a nucleotide analog relative to the natural substrate, or by increased phosphorolytic
cleavage of an analog-blocked primer. To overcome these acquired resistances, the design of the
next generation of NRTIs has been mainly focused on two fronts: (i) nucleoside analogs possessing
a 3
′
-5
′
hydroxyl group that can induce delayed polymerization arrest; (ii) nucleotide analogs that are
designed to be incorporated into the viral genome during replication. These nucleotide analogs can
introduce mutations into the HIV genome through mispairing and blockade of the replication process.
′
11.4.1.2 Human Steroid 5
a
-Reductase Inhibitors
The human enzyme steroid 5
-reductase is responsible for the conversion of testosterone (T)
to the more potent androgen, dihydrotesterone (DHT). It has been shown that abnormally high
5
α
-reductase activity in humans leads to excessively high DHT levels in peripheral tissues.
Inhibition of 5
α
-reductase thus offers a potential treatment for DHT-associated diseases, such as
benign prostate hyperplasia, prostate cancer, acne, and androgenic alopecia. In humans, there are
two types of steroid 5
α
-reductase is mainly expressed
in the sebaceous glands of skin and the liver, while the type II enzyme is most abundant in the
prostate, seminal vesicles, liver, and epididymis. The i rst 5
α
-reductase: type I and type II. The type I 5
α
-reductase inhibitor approved for
clinical application in the United States was i nasteride; it is currently employed in the treat-
ment of benign prostatic hyperplasia (BPH) in men. This compound is approximately 100-fold
more potent toward the type II than the type I isozyme of 5
α
-reductase. In humans, i nasteride
decreases prostatic DHT levels by 70%-90%, resulting in reduced prostate size. The detailed
biochemical characterization of i nasteride inhibition suggested that i nasteride is a mechanism-
based inhibitor. It is proposed that by closely mimicking the substrate (testosterone), i nasteride is
accepted as an alternate substrate and forms an NADP-dyhydroi nasteride adduct at the enzyme
active site (Figure 11.4). This covalent NADP-dyhydroi nasteride adduct represents a bisubstrate
analog with extremely high afi nity (
K
i
α
≤
1 × 10
−13
M) to the type II 5
α
-reductase. Interestingly,
i nasteride is also a mechanism-based inhibitor of the human type I 5
α
-reductase. However, the
NADP-dyhydroi nasteride adduct formation rate at the type I 5
-reductase active site is reduced
by more than 100-fold compared to that for the type II isozyme. This difference in NADP-
dyhydroi nasteride adducts formation rate accounts for the isozyme selectivity of i nasteride both
in vitro
and
in vivo
. Knowledge of the mechanism of inhibition of 5
α
α
-reductase by 4-azasteroids
(represented by i nasteride) and of the SAR for dual 5
α
-reductase inhibition, led to the discovery
of a potent, dual inhibitor of 5
α
-reductase, known as dutasteride. Dutasteride, is equipotent versus
type I and type II 5
-reductase and demonstrates exceptional
in vivo
potency. This compound has
also been approved for clinical use in the treatment of BPH.
α
11.4.2 I
NTERMEDIATE
S
TATE
-B
ASED
D
ESIGN
11.4.2.1 Inhibitors of Hydroxymethylglutaryl-CoA Reductase (HMG-CoA Reductase)
The biosynthetic pathway for cholesterol involves more than 25 different enzymes. The enzyme
3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase catalyzes the conversion from
