Download Lecture 22-Lutz

Prostaglandins:
Synthesis, functions and
inhibitors
Carol S. Lutz, Ph.D.
Lecture 23
March 16, 2015
Objectives:
•Discover and distinguish relationships between
different types of eicosanoid molecules
(PGs, TXs, LTs)
•Gain knowledge of enzymes that create PGs
and TXs (COX-1 and -2)
•Explore physiological functions of these molecules
•Ascertain how pharmacological inhibitors work to
block activity
Prostaglandins and related compounds
(thromboxanes and other eicosanoids):
- Potent cell signalling molecules –
paracrine hormones
- Multiple effects, including pain and
inflammation associated with
arthritis
Synthesized from essential dietary
fatty acids, esp. linoleic acid
Prostaglandins:
-discovered in the 1930s by Ulf von Euler
-synthesized in virtually every cell in the body
-unsaturated 20 carbon molecules,
also contain a 5-member ring
-work right within the cells where they
are synthesized—”act locally”
-have an extremely short half-life and are
not stored
Examples of prostaglandin
structures
-note the similarities and
differences
Lippincott Fig 17.21
The first step in the synthesis of
prostaglandins is the oxidative
cyclization of free arachidonic acid
to yield PGH2 by prostaglandin
endoperoxide synthase. PGH2
is converted to a variety of
prostaglandins and thromboxanes
by a variety of cell-specific
synthases.
Note: Two catalytic activities
Lippincott 17.22
Cyclo-oxygenase Isoforms
(COX-1 vs COX-2)

Two isoforms of COX
–

COX-1 is constitutive, expressed in most tissues
–
–
–

Both produce prostaglandins (PGE2, PGF2a, PGI2)
physiological and homeostatic role, cell signalling
not influenced by steroid administration
not increased by cytokines nor bacteria
COX-2 is inducible following inflammation,
trauma, etc
–
–
–
–
found in immunocompetent cells (e.g. leukocytes)
pathophysiological role, maintains inflammation
induced by cytokines (interleukin-1)
inhibited by steroids
THE COX GENES, mRNAs, PROTEINS
The pathway of prostaglandin
production, revisited, with
insights into
pro-inflammatory regulation
Another view………………..
or linoleic acid
PGE2 plays an important role in the development,
regulation, and activity of different cells of the
immune system……..
Prostaglandins play an integral role in a myriad of infections
and diseases, including cancer…
…and periodontal disease......
Functions of prostaglandins
1. Activation of the inflammatory response, production of pain,
and fever. When tissues are damaged, white blood cells are
mobilized to the site to minimize tissue destruction.
Prostaglandins are produced as a result.
2. Blood clots form when a blood vessel is damaged. Closely
related molecules called thromboxanes stimulate constriction
and clotting of platelets. Conversely, PGI2 is produced to have
the opposite effect on the walls of blood vessels where clots
should not be forming.
Functions of prostaglandins, cont.
3. Certain prostaglandins (i.e. PGE2) are involved with induction
of labor by inducing uterine contractions.
4. Prostaglandins are involved in several other organs
-regulate salt and fluid balance in body
-increase blood flow in kidneys
-increase secretion of protective mucus in GI tract
-inhibit acid synthesis in GI tract
-leukotrienes, related molecules, promote constriction of
bronchi associated with asthma
The inhibitors….
How do NSAIDs work?
Summary
• Inhibit COX enzymes
• Reduce production of prostaglandins and thromboxanes
• Reduce pain, fever and inflammation
But…
• Side effects of some NSAIDs in some people
- include gastrointestinal irritation, ulcers,
bleeding, etc.
- if untreated, lead to death (~16,000/yr in US)
- led to hunt for better anti-inflammatory drugs
Effects of aspirin and other pain killers?
Aspirin works on both COX-1 and COX-2 to inhibit
arachidonic acid’s entry into the active site of the enzyme
--acetyl group of aspirin binds to serine in COX
-- by blocking the activity of the COX enzymes, this
relieves some of the effects of pain and fever
--”nonselective”
--many side effects
Tylenol—thought to have effects through inhibiting
the activity of COX-3, an alternatively spliced form
of COX-1
Low dose aspirin therapy:
-Aspirin irreversibly inhibits COX-1 (and COX-2)
-Has a short half-life
-New platelets are constantly being made
-Will therefore reduce but not abolish the ability
for blood to clot, thereby reducing heart
attacks and stroke
Short History of COX-2 Inhibitors (1)
• 1970’s - aspirin target identified as cyclo-oxygenase
(COX)
• 1980’s - Two forms of COX identified:
COX-1, constitutive
COX-2, induced at sites of inflammation
- Hypothesis: selective inhibition of COX-2 might
reduce inflammation without the GI side effects.
• 1990’s - Rational design of COX-2 inhibitors
Rational Drug Design (1)
-
Based on molecular and structural studies, informed by cell
biology, physiology, pharmacology, etc.
Rational Drug Design (2)
Non-selective NSAIDs
COX-1
COX-2
Selective COX-2 inhibitors
COX-2 inhibitors
Coxibs – a new class of NSAIDs
Merck
Pfizer
Vioxx (rofecoxib) – withdrawn from sale by Merck,
September 30, 2004
Arcoxia (etoricoxib) – successor to Vioxx? Not (yet) approved
in US
Celebrex (celecoxib) – FDA recommended ‘black box’ warning
label
Bextra (valdecoxib) –withdrawn from the market, 2005
Short History of COX-2 Inhibitors (2)
• mid 1990’s - X-ray crystal structures showed that COX-2 active site has a side
pocket that is absent in COX-1.
• late 1990’s - inhibitors designed that preferentially bind COX-2, are equally
effective as NSAIDs, but are reported to cause less GI damage.
• 1998/1999 - Celebrex and Vioxx approved by FDA for osteoarthritis and
rheumatoid arthritis.
• 2000’s
- next generation of drugs developed with even higher selectivity
for COX-2 over COX-1.
Triumph for rational drug design
Freedom from GI side effects make the COX-2
inhibitors very attractive…….
Short History of COX-2 Inhibitors (3)
How the problems came to light
June 2000-VIGOR trial-compared Vioxx to naproxen-could not tell if
Vioxx was bad or naproxen was protective
Sept. 30, 2004 - APPROVe trial (Adenomatous Polyp Prevention on Vioxx)
led Merck to withdraw Vioxx voluntarily.
Randomized, placebo-controlled trial (2606 patients, for Vioxx use in
preventing colorectal polyps) showed an increased risk of heart attack
and stroke.
The increased risk is now estimated as 2.3-fold (from a meta-analysis).
Dec. 20, 2004 - APC trial (Adenoma Prevention by Celecoxib) by
Pfizer reported ~ 2.5-fold increase in major fatal
or non-fatal cardiovascular events.
Summary
•Prostaglandins, thromboxanes and leukotrienes are known
as eicosanoids
•They are produced in small amounts in almost all tissues,
act locally, and have many important physiological and
pathological functions
•The dietary precursor of eicosanoids is the essential fatty acid,
linoleic acid
•Synthesis of prostaglandins begins with oxidative cyclization
of free arachidonic acid to yield PGH2 by prostaglandin
endoperoxide synthase (cyclooxygenase)
•There are two isozymes of the synthase: COX-1 and COX-2,
which are important drug targets