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The aforementioned in vitro studies established a predominant role for CYP3A4
in the metabolism of nirmatrelvir (1). In order to overcome potentially suboptimal
therapeutic exposures, nirmatrelvir (1) is co-dosed with the potent CYP3A4 inactivator
ritonavir (2) as a pharmacoenhancer to boost its therapeutic concentrations.
In the past, pharmacoenhancers have been adopted for multiple purposes such as
ensuring therapeutic exposure of the active product, reducing formation of toxic
metabolites, changing the route of administration and increasing the cost-effectiveness of
a therapy. Ritonavir (2) has been used as a pharmacoenhancer of several marketed
protease inhibitors (e.g., darunavir and lopinavir) that are subject to metabolic clearance
through CYP3A4. It is also a permeability-glycoprotein (P-gp) inhibitor, which further
elevates the bioavailability of nirmatrelvir (1). It substantially elevates blood levels of coadministered CYP3A-dependent drugs, with increases in area-under-the-curve (AUC)
blood concentrations ranging from 1.8- to 20-fold. In this case, co-administration with
ritonavir (2) significantly improved the plasma concentration of nirmatrelvir (1).
On the other hand, ritonavir (2) is an inducer of several other CYP enzyme
subtypes such as CYP1A2, CYP2B6, CYP2C9, and CYP2C19, as well as UDPglucuronyl transferases. Thankfully, since induction occurs slowly (i.e., generally 10–15
days after initiation of inducer), it is not anticipated to cause significant drug–drug
interactions (DDIs) as an inducer because of its short treatment course of 5 days.
Chemistry and Pharmacology of Drug Discovery
15
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5. Efficacy and Safety
Paxlovid [two 150 mg of nirmatrelvir (1)/one 100 mg of ritonavir (2)] reduced the risk of
hospitalization or death by 89% in high-risk individuals who took the drug within 3 days
of experiencing symptoms.
In a randomized clinical trial (EPIC-HR) with 1 : 1 Paxlovid: placebo received
orally every 12 h for 5 days, it was found that those receiving Paxlovid had significantly
reduced hospital admissions and deaths among those affected by COVID-19. It was
found that among participants who received treatment within 3 days of beginning
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Chapter 1. Nirmatrelvir (Paxlovid)
COVID-19 symptoms, the risk of hospitalization or death was 89% lower than that for
17,18
the placebo group, without evidence of safety concerns.
Paxlovid’s safety has also been established.
In vitro, nirmatrelvir (1) was remarkably selective against receptors, ion
channels and other enzymes. It was also selective against other coronaviruses such as
SARS-CoV-1, MERS-CoV and human coronavirus 229E using cytopathic effect (CPE)
assays to measure their cellular antiviral activities. The drug was not a hERG substrate
thus had a low potential for cardiotoxicity. Nirmatrelvir (1) was not mutagenic or
clastogenic in in vitro genetic toxicity studies and was negative in an in vivo rat
micronucleus assay. Finally, it was well tolerated in long-term toxicity tests in monkey
10
with no observed adverse effects at high doses.
Also in the EPIC-HR clinical trial, the incidence of adverse effects that emerged
during or after the treatment period (treatment emergent adverse effects, TEAEs) was
similar among recipients of Paxlovid (22.6%) and the placebo (23.9%). Most TEAEs in
both treatment groups were mild to moderate (Grades 1–2) in severity. No patients in the
Paxlovid group experienced an AE resulting in death (Grade 5), while there were 13
deaths among placebo recipients.
17,19
Now that millions of doses of Paxlovid have been taken in real world. Its
efficacy and safety have been well established.
6. Synthesis
The first 7 mg of nirmatrelvir (1) were synthesized in late July 2020. A massive scale up
effort was undertaken and by late October 100 g of nirmatrelvir (1) was synthesized and
2 weeks later the chemists were able to scale up the synthesis to even more than 1 kg.20
6.1. Scale-up Route
We first review the route that Pfizer used to make their 150-g batch.10
The pyrrolidone-containing fragment at P
ester 20. A simple amination of 20 converted the ester to primary amide 21, which was
treated with HCl to remove the Boc protection, furnishing the P
HCl salt.
was prepared using known amino-
1
fragment as 22 as its
1
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Chemistry and Pharmacology of Drug Discovery
Meanwhile, dimethylcyclopropylproline ester 23 was coupled with (S)-tertleucine 24 to assemble amides 25. Hydrolysis of the ester functionality on 25 using
lithium hydroxide then provided acid 26, which was treated HCl to afford the “naked”
amino-acid 27 as its HCl salt. Condensation of 27 with ethyl trifluoroacetate led to the
formation of trifluoroacetamide 28. Another key coupling between 28 and 22 assembled
dipeptide 29, which was readily dehydrated to deliver nirmatrelvir (1) as its methyl t-
butylether (MTBE) solvate after recrystallization using a mixture of MTBE and ethyl
10
acetate.
Since each step has a high yield, the overall yield is 48% to make nirmatrelvir
(1) from lactam 22.
Boc protection/deprotection (twice!) and the employment of the Burgess reagent
as the dehydrating reagent to make the nitrile are not amenable to commercial-scale
production, which mounts to hundreds of tons of the API. Pfizer development chemists
developed better synthesis for large-scale synthesis. Before disclosure of Pfizer’s
manufacturing route, we highlight a few examples among numerous efforts at optimizing
the production of nirmatrelvir (1).
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Chapter 1. Nirmatrelvir (Paxlovid)
A group at Virginia Commonwealth University chose to use trifluoroacetic
anhydride (TFAA) to replace expensive Burgess reagent, which also left behind a lot of
by-product waste. As shown below, treating primary amide 29 with 2 equiv of TFAA and
2 equiv of N-methylmorpholine (NMM) delivered nirmatrelvir (1) as its methyl t-
butylether (MTBE) solvate in 85% yield after recrystallization using a mixture of MTBE
and ethyl acetate. The group also employed propanephosphonic acid anhydride (T3P) as
the obvious replacement of Pfizer’s coupling agents, HATU and EDCI, with good
21
results.
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Chemistry and Pharmacology of Drug Discovery
In 2023, a group in India published a preparation of nirmatrelvir (1) using flow
chemistry. They chose to make the nitrile sooner than Pfizer did. To that end, dehydration
of primary amide 21 with TFAA to give nitrile 30, which was treated with HCl to
produce the P
and acid 28 using T3P as the coupling agent delivered nirmatrelvir (1) directly.
fragment as amine 31 as its HCl salt. Finally, coupling between amine 31
1
21
It could be advantageous to make the nitrile sooner than doing it at the last step.
Also in 2023, a group in China published an optimized synthesis of a key
intermediate of nirmatrelvir, primary amide 21. Thus, deprotonation of dimethyl N-
BocGlu (32) using LiHMDS was followed by α-cyanomethylation with bromoacetonitrile
in the presence of NdCl
group with concomitant cyclization and ammonolysis and subsequent deprotection of N-
. A one-pot Raney nickel-catalyzed hydrogenation of the nitrile
3
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Boc to deliver the target intermediate cyclic glutamine analog in three steps in high
22,23
yields.
This route reduced side reactions and undesired by-products. The operations are
simple. But using LiHMDS is not my favorite feature.
Chapter 1. Nirmatrelvir (Paxlovid)
6.2. Manufacturing Route
In 2023, Pfizer Process Chemistry published their manufacturing route for nirmatrelvir
(1), which took 17 months from first laboratory synthesis (in July 2020) to FDA
emergency use authorization (in December 2021).
The manufacturing route starts over 100 metric tons of starting raw materials, so
the cost and even supply chain considerations were paramount, especially during the
COVID pandemic.
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Starting material 23 was prepared by Merck/Codexis before for their HCV NS3
serine protease inhibitors program to make boceprevir (8). The HCl salt of 23 was at first
basified with triethylamine to release the free amine and the methyl ester was converted
to its corresponding sodium salt 34 by the treatment of NaOH. Treating acid 35 with
mesylate gave rise to a mixed anhydride, which was coupled with 34 to assemble 28. The
coupling between acid 28 with amine 31 was accomplished by EDAC to provide adduct
29 with the aid of 2-hydroxypyridine N-oxide (HOPO), which has been known to reduce
racemization. Treating primary amide with TFAA afforded nitrile 36 as the MTBE
solvate, which was converted to anhydrous Form I of nirmatrelvir (1).
Chemistry and Pharmacology of Drug Discovery
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7. Summary
The discovery and development of nirmatrelvir (1)/Paxlovid has been an important
chapter in the annals of pharmaceutical industry. The drug literally saved lives.
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Pfizer’s triumph has been built on decades of progress made in the field of
protease inhibitors. As we see throughout this chapter, the pyrrolidone substituent at P
the rigid bicyclic dimethylcyclopropylproline at P
been precedented. Needless to say, ritonavir (2) as a PK enhancer was discovered in the
1990s when the first wave of HIV protease inhibitors became available.
After all, if we have seen further, it is by standing on shoulders of giants.
Chapter 1. Nirmatrelvir (Paxlovid)
and the nitrile warhead at P1′ have all
2
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