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):

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- Potent cell signalling molecules – paracrine hormones

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- Multiple effects, including pain and inflammation associated with arthritis

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Synthesized from essential dietary fatty acids, esp. linoleic acid

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Prostaglandins:

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-discovered in the 1930s by Ulf von Euler

-synthesized in virtually every cell in the body

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-unsaturated 20 carbon molecules,

also contain a 5-member ring

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-work right within the cells where they

are synthesized—”act locally”

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-have an extremely short half-life and are

not stored

Examples of prostaglandin

structures

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-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.

Lippincott 17.22

Note: Two catalytic activities

Cyclo-oxygenase Isoforms �(COX-1 vs COX-2) �

• Two isoforms of COX
• Both produce prostaglandins (PGE₂, PGF_2α, PGI₂)

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• COX-1 is constitutive, expressed in most tissues
• physiological and homeostatic role, cell signalling
• not influenced by steroid administration
• not increased by cytokines nor bacteria

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• 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

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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.

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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.

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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

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• Reduce production of prostaglandins and thromboxanes

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• 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:

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• 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)

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• 1980’s - Two forms of COX identified:

COX-1, constitutive

COX-2, induced at sites of inflammation

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- Hypothesis: selective inhibition of COX-2 might

reduce inflammation without the GI side effects.

• 1990’s - Rational design of COX-2 inhibitors

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Rational Drug Design (1)

• Based on molecular and structural studies, informed by cell biology, physiology, pharmacology, etc.

Rational Drug Design (2)

Non-selective NSAIDs

Selective COX-2 inhibitors

COX-1

COX-2

COX-2 inhibitors

Coxibs – a new class of NSAIDs

Merck

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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

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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.

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• late 1990’s - inhibitors designed that preferentially bind COX-2, are equally

effective as NSAIDs, but are reported to cause less GI damage.

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• 1998/1999 - Celebrex and Vioxx approved by FDA for osteoarthritis and

rheumatoid arthritis.

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• 2000’s - next generation of drugs developed with even higher selectivity

for COX-2 over COX-1.

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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

Sept. 30, 2004 - APPROVe trial (Adenomatous Polyp Prevention on Vioxx)

led Merck to withdraw Vioxx voluntarily.

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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).

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Dec. 20, 2004 - APC trial (Adenoma Prevention by Celecoxib) by

Pfizer reported ~ 2.5-fold increase in major fatal

or non-fatal cardiovascular events.

June 2000-VIGOR trial-compared Vioxx to naproxen-could not tell if

Vioxx was bad or naproxen was protective

Summary

• Prostaglandins, thromboxanes and leukotrienes are known

as eicosanoids

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• They are produced in small amounts in almost all tissues,

act locally, and have many important physiological and

pathological functions

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• The dietary precursor of eicosanoids is the essential fatty acid,

linoleic acid

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• Synthesis of prostaglandins begins with oxidative cyclization

of free arachidonic acid to yield PGH₂ by prostaglandin

endoperoxide synthase (cyclooxygenase)

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• There are two isozymes of the synthase: COX-1 and COX-2,

which are important drug targets