Introduction
Drug discovery
and development is a long process and very expensive with a relatively short
window of patent protection during which to recoup the investment and actually
make some money. The patents are designed to cover the drug as broad as
possible to prevent someone else from making a simple small change to the
molecule and bypassing the patent protection.
A good
patent covers the drug, its indication and ways of making it, its metabolites,
its different salts, crystal forms, methods of delivery, dosage, combination
therapy, etc etc.
Pretty
rigorous, wouldn’t you agree? Well, there are some nice examples out there
where someone could have done some more homework on the drug’s metabolism and
made the company much more money.
Example 1 –
Benzodiazepines
In the 1960s,
Geigy launched Carbamazepine [1] as an anticonvulsant and mood-stabilizing drug
for epilepsy and a few other disorders. [2] It is still being prescribed
(Tegretol), however, it has some side effects. The main liver metabolite of
Carbamazepine is Oxcarbazepine which has less side effects and is sold by Novartis [3]. By that time,
Geigy was part of Novartis, so I guess the revenues went to the same coffers.
The main metabolite of Oxcarbazepine, and the real active component, is its monohydroxy
derivative Licarbazepine “MHD”, which is essentially just a reduced form. [4] Clever
people at Sepracor figured out that the S-enantiomer worked better than the
racemate [5] and launched Eslicarbazepine. Since it was shown that the
S-enantiomer was the preferred drug, the Portugese company Bial developed
Eslicarbazepine acetate as a pro-drug. They teamed up with Eisai and Sepracor
for the development and marketing.
Figure 1: Better
and better metabolites as marketed drugs. Finally, a pro-drug to bypass some
IP.
Hydroxyzine
has been around for >60 years and is still used today.[6] Hydroxyzine is metabolized
in the liver to Cetirizine which became a second-generation anti-histamine. [7]
Again, it was found that only one of the enantiomers is the most active form:
Levo-cetirizine. In this case, the company UCB has been successful in
re-inventing the drug two times, now marketing this third-generation
anti-histamine.
Figure 2:
Three generations anti-histamines. We could have had Gen 3 already in the
50-ies.
Example 3 –
Ezetimibe
Another drug
where an active metabolite played a large role in the development is Ezetimibe
(Schering Plough).[9-11] As can be seen in Figure 3, the initial hit that came
from high throughput screening is a relatively undecorated molecule. While
testing the candidate for effectiveness in the rat, the researchers found that
the bile of the rat contained active metabolites that were more effective. Structure
elucidation showed introduction of a polar group on the α-position, the
(S)-alcohol being the most interesting.
A thorough
follow-up metabolism study showed improved pharmacokinetic behavior when the
methoxy groups were removed from the scaffold and when certain sites were
blocked from P450 metabolism by exchanging hydrogen for fluorine atoms.
Figure 3:
Development of lead to drug, the Ezetimibe story in one picture.
Discussion
The above
examples clearly show the importance of rigorous study of metabolism of the
drug candidate. The usual tools that are available are 1) human and animal liver fractions like
microsomes and hepatocytes, 2) the individual CYPs, 3) chemical methods of
oxidizing molecules to mimic human metabolism.
Especially the
chemical methods are rather crude since they mostly do not have any selectivity
and always oxidize the easiest positions first. There are porphyrin systems
that are designed to resemble a P450 that show some promise (I guess anything
is better than bleach) but these alternatives cannot come close to the rich metabolism
that enzymes can achieve. [12]
As such, it
is surprising that the pharmaceutical industry has not embraced the diversity that
is out there in Nature. There are many bacterial P450 enzymes that show overlap
with human metabolism, yet also offer the distinct opportunity at making new
non-human metabolites. The most studied and easy-to-use bacterial P450 enzyme
is the CYP102A1 from Bacillus megaterium
(“BM3”). In 2009, researchers from Eli Lilly and CalTech have written a very
nice paper on the testing of the hypothesis that bacterial P450s have a place
in drug development. [13] Figure 4 below is one of their examples, Verapamil, that
shows that all major human metabolites are formed by a small set of mutant
enzymes, as well as four new metabolites.
Figure 4:
Verapamil metabolism by human CYPs (black box) and the broader BM3 metabolism
(red box).
Conclusion
I don’t know
about you, but I keep wondering what the Ezetimibe drug would have looked like
if bacterial P450s had been used in the development.[14] Maybe the drug’s efficacy
would be better if oxidation had occurred at the beta or gamma positions. This
was not tested since there is no animal that displays that metabolism….
References
[1] My good
friend Martin Mayhew (now at Verenium; www.linkedin.com/pub/martin-mayhew/b/4b7/673)
commented on my previous post, mentioning that CYPs also can have synergistic
and/or antagonistic effects, as another reason to do the microsome testing
alongside individual CYP testing. For instance, Carbamazepine is metabolized by
CYP 3A4 but induces (= “make more active”) other CYPs and this has an effect on
other drugs if they are used together with Carbamazepine.
[2] http://en.wikipedia.org/wiki/Carbamazepine;
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000620/
[5] A
racemic mixture is a 1:1 mix of the enantiomers (mirror images, like left hand
and right hand; http://en.wikipedia.org/wiki/Enantiomers
). See here for a nice paper on the two enantiomers: http://www.ncbi.nlm.nih.gov/pubmed/20578208.
Sepracor became known for this trick, which was called “chiral switch”: http://www.usm.edu/phillipsgroup/CHE255_S10/chapter5/Chirality_at_work.pdf
[9] SB.
Rosenblum , T Huynh , A Afonso , HR Davis , Jr., N Yumibe , JW. Clader, and DA.
Burnett. Discovery
of 1-(4-Fluorophenyl)-(3R)-[3-(4-fluorophenyl)-(3S)-
hydroxypropyl]-(4S)-(4-hydroxyphenyl)-2-azetidinone (SCH 58235): A Designed, Potent, Orally Active Inhibitor of
Cholesterol Absorption. J. Med. Chem., 1998, 41 (6), pp 973–980.
[10] Clader
JW. The discovery of ezetimibe: a view from outside the receptor. J. Med. Chem.,
2004, 47 (1), pp 1-9.
[12] For
instance: K Inami, M Mochizuki. Chemical models for cytochrome P450 as a
biomimetic metabolic activation system
in mutation assays. Mutat Res. 2002, 519(1-2), pp 133-40. E Brulé, YR de Miguel.
Supported manganese porphyrin catalysts as P450 enzyme mimics for alkene
epoxidation. Tetrahedron Letters 2002, 43 (47), pp 8555–8558.
[13] AM Sawayama, MMY Chen, P Kulanthaivel, MS Kuo, H Hemmerle, and FH Arnold. A Panel
of Cytochrome P450 BM3 Variants to Produce Drug Metabolites and
Diversify Lead Compounds. Chem. Eur. J., (2009) 15, pp 11723–11729.
[14] Codexis
sells these as licensed from CalTech: http://www.codexis.com/pdf/Codexis_MycroSys.pdf.
They’re called MicroCYPs although you would not say so from the file name :-(.
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