The Promise of a Non-addictive Opioid

As far back as the late 1800s heroin was once hailed with the promise of being an abuse-free opiate. Although dozens of other opiates have been created, no one in the past 12o years has succeeded in creating a side-effect-free and non-addictive opioid. But by using a new approach, there is a chance that a German team led  by the anesthesiology and professor Dr. Christoph Stein has succeeded. Before delving into the specifics and exploring the chemistry, let’s provide some background information.

What Are Opioids?

An opioid can be any of these compounds: (1) it can be part of the group of substances directly made by, or derived from, the opium poppy; (2) it can be one of the human-brain-produced peptides that influence a variety of  behaviours including attachment, stress, food-intake and pain response; or (3) it can be a synthetic substance that binds to the same receptors as the previous two types. The receptors known so far include μ (mu—the m coming from “morphine”), δ (delta) and κ (kappa).

Opioids are used to treat pain brought on by terminal or serious illnesses, such as cancer. Some medical practitioners also turn to opioids for chronic pain, common among those with forms of arthritis, back injuries, torn muscles, damaged nerves and fibromyalgia. A case of kidneys stones, nature’s way of making men experience something almost as painful as childbirth, sure made me appreciate a dose of morphine. Since that experience, I have not needed a strong opiate.

Others are not as lucky and need more intense compound-assisted pain management, especially if other opioids have become ineffective. Fentanyl (trade names: Actiq®, Duragesic®, and Sublimaze®) is a quick-acting synthetic opioid with 50-100 times the strength of morphine. But transdermal patches of fentanyl are prescribed only when at least a week of tolerance towards opiates has already been established. That’s because with all its delivery-forms, the use of fentanyl can be very risky. Opioid receptors are all over the body, brain included, and fentanly will not only cause dopamine levels to spike, but it could also bind to brain receptors that control the breathing rate. Respiratory depression can be fatal, and the online availability of illicit fentanyl sold through decoy packages has led to an epidemic rise in fentanyl-related deaths in Canada, Estonia and the United States.

WEB_urinestrips
The first pouch on the right hides a few grains of fentanyl (enough to kill). The package was mailed from China pretending to be a set of urine-testing strips. From the Globe and Mail

Other undesirable reactions from binding to μ receptors include reduced gastrointestinal nausea, vomiting, and the accompanied euphoria from the ensuing dopamine-spike, which ties into addiction.

How A Safer Alternative Was Found

The researchers started by focusing on the role of MOR (μ opiate receptors) in injured tissues as opposed to the brain. They did this for two reasons (1) Painful conditions such as inflammation or trauma are often associated with a lower pH(acidification) in localized tissue. (2) Only the protonated form of opiates like fentanyl can bond to MOR receptors. To be protonated simply means that an H+ ion has been attached to a basic group, in this case the nitrogen of a heterocyclic ring called piperidine . Then through hydrogen bonding, that proton attracts the oxygen of a carboxylate group of an aspartic acid molecule on the receptor.

fentanylCapture
The protonated fentanyl shown bonded to an aspartic acid residue (Asp 147) of a μ opiate receptor. Modified from author’s paper in Science.

Fentanyl and other opiates have substantial fractions of their molecules protonated at pHs of about 5 to 7, where the source of pain lies.  But the problem is that opiates are also highly protonated at pH 7.4 , the typical pH of the brain and small intestines where you don’t want the opiates to be activated.

We will represent the protonated form of fentanyl as HNFen+ and the uncharged basic counterpart as NFen:

HNFen+ = H+ + NFen

What the researchers aimed to do was to attach a fluorine group to key positions of the fentanyl molecule to increase its acidity without altering the stereochemistry or functional groups of the rest of the molecule. After some computer modelling they suggested the synthesis of NFEPP:

Nfepp
The protonated form of NFEPP, which has been shown to be a non addictive opioid still capable of acting as an analgesic. This form becomes less important above a pH equal to its pKa of 6.8
fentanyl
The protonated form of fentanyl found in the commercial citrate. It’s also the predominant form below pH = pKa = 8.4, so in other words in any human tissue containing opioid receptors.

Even at the beta position (on the second adjacent carbon attached to nitrogen), fluorine’s electron-withdrawing presence decreases a nitrogen’s ability  to reattach itself to H+. This happens because nitrogen uses its electron pair to act as a base and bond to H+. If the reverse reaction is less likely to proceed, the acidic forward reaction is favoured. That is a good thing for the drug designers because they did not want the protonated form to dominate in certain tissues where it could lead to addiction and respiratory arrest.

To understand why a more acidic molecule will be adequately protonated in the proton-rich environment —where the pain is—and why there will be far less protonated than fentanyl at pH=7.4, let’s explore some analytical chemistry.

For weak acid systems like

HNFen+ = H+ + NFen , we cannot make the assumption that the H+ concentration will equal that of NFen. To estimate the relative concentration of the protonated fentanyl  we have to rely on the mass balance approach where we compare HNFen+ ‘s concentration, [HNFen+ ], to that of the total of  non H+ species.

We let X = [HNFen+ ]+ [NFen] = sum of concentration of non-H+ species.

so [NFen] = X – [HNFen+ ]

since its acid dissociation constant, Ka = [H+][NFen]/[HNFen+ ], by susbtituting for [NFen] we obtain:

Ka = [H+](X – [HNFen+] )/[HNFen+].  By then solving for [HNFen+ ], we obtain:

[HNFen+] =  [H+]X / ( Ka + [H+] )

Finally by rearranging the expression, we get the fraction of protonated species as:

[HNFen+]/X = [H+]/ (Ka + [H+] )

The expression suggests that the fraction of the protonated species is pH-dependent and is inversely proportional to the strength of the acid (measured by its Ka). If [H+] = Ka or if pKa = pH , we will get a mole fraction of exactly of 0.5. This implies that since NFEPP’s pKa is lower (6.8) than fentanyl’s, then the concentration of the protonated form will be the minority-species at a pH that’s lower than the crossover point for fentanyl. The latter has a lower Ka and hence higher pKa of 8.4. To better reveal the relationship, I plotted the fraction [HNFen+]/X versus the pH for both fentanyl and for NFEPP, which due to its fluorine has a higher Ka.

A-pHplot
Notice how the fraction of the protonated form of NFEPP drops dramatically to about 10% at pH= 7.4 whereas that of fentanyl is still above 90%. Meanwhile NFEPP still acts as an analgesic because at pHs of 6.8 to 5.2, the fraction of the protonated form that’s needed to bond to μ receptors in inflammation areas is in the 52 to 95% range, respectively.

But does all this theory work in a biological environment? In the authors’ experiments, NFEPP produced analgesia in rats who had different types of inflammatory pain. And equally important the pain relief was not accompanied by typical opiate side effects such as sedation, decreased breathing, constipation or the desire to seek more pain-killer. In 4 to 5 years, the time it will take to refine the synthesis and test the product in humans, it will be interesting to see if the usual unforeseen consequences will be minor. Of course NFEPP ‘s inability to produce euphoria will not displace dangerous opiates from the black market. But it will help alleviate the problem by putting nonaddictive drugs into circulation while hopefully making the legitimate production and prescription of fentanyl and other opiates obsolete.

Sources:

A nontoxic pain killer designed by modeling of pathological receptor conformations http://science.sciencemag.org/content/355/6328/966

Fluorine in Medicinal Chemistry ChemBioChem 2004, 5, 637 ± 643 https://ccehub.org/resources/611/download/637_ftp.pdf

http://consumerhealthchoices.org/wp-content/uploads/2012/08/BBD-Opioids-Full.pdf

A brief history of opiates, opioid peptides, and opioid receptors Michael J. Brownstein Laboratory of Cell Biology, National Institute of Mental Health, Bethesda, MD 20892  http://www.pnas.org/content/90/12/5391.short

http://www.sciencedirect.com/science/article/pii/S0304395904004476

https://www.drugabuse.gov/publications/drugfacts/fentanyl

http://www.theglobeandmail.com/news/investigations/a-killer-high-how-canada-got-addicted-tofentanyl/article29570025/

http://www.theglobeandmail.com/news/national/estonias-cautionary-fentanyl-tale-for-canada/article30512811/

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Carfentanil has killed hostages and even moose!

carfentanil
Source: Vancouver, Royal Canadian Mounted Police

Carfentanil is a potent opioid, one of many molecules from the Pandora’s box of organic chemistry labs.  Derived from fentanyl, another synthetic morphine-like compound,  carfentanil has been deliberately introduced into samples of  heroin. Frontline emergency responders have been advised to wear masks and gloves to avoid ingesting minute amounts of the drug. It’s also been described as a substance designed to knock out elephants.

I will describe a couple of related stories from the research literature that have not been mentioned in the news recently, and of course since unlike newspapers, I don’t mind losing readers by dwelling on chemistry,  I will also take a closer look at the molecule. 🙂

hostageOn October 23, 2002, more than 800 people attending a stage show in Moscow were taken hostage by about 50 Chechen rebels. After the latter threatened to blow up everyone in there if their political demands were not met, 3 days later, the Russian military stormed the theatre. To subdue the rebels, an unidentified “gas” was introduced through the ventilation system about 15 minutes before the soldiers moved in.  Things however did not go according to plan. After the “rescue”, the hostages had to be taken to the hospital. Due to wrong assumptions about the nature of the “gas”, many patients were given ineffective treatments and died. But a couple of doctors noticed that the patients showed classic signs of opioid intoxication: pinpoint pupils, unconsciousness and depressed breathing. The opioid hypothesis was supported by the fact that naloxone was able to save those who received the same drug that’s typically used in opioid-related emergencies. Later, the Russian health minister tried to assure the public that the  mixture used during the military intervention was not lethal. They admitted that the aerosol included an opioid, specifically a fentanyl-derivative (likely carfentanil). But convinced of the opioid’s harmlessness, they tried to pin the blame for all of the 127 deaths on the conditions experienced by hostages during the 3 days of captivity.

What else was in the opioid mixture? German chemists found it contained halothane, an anaesthetic, but that wasn’t the fatal ingredient. According to a report of three medical toxicologists from the Good Samaritan Regional Medical Center, the problem was rooted in the Russians’ underestimation of carfentanil’s potency. They had extrapolated rat models that did not apply to humans.  In addition, the Russians didn’t take into account that a ventilation system will not evenly distribute an aerosol.

What kind of a job does carfentanil do on large mammals? The Quebec Environment Ministry did a 10-year study in the late 1980s to 1990s involving carfentanil and free- ranging moose. A dose of 3 mg of carfentanil was mixed with xylazine and injected into 69 different animals. The xylazine was included to smooth out induction and recovery.  Although 3 mg represents only only 7.1 parts per billion for moose with an average mass of 422 kg, most animals slowed down in only 6 minutes and were immobilized in half an hour. Four moose however died. So if in reading the news you became under the impression that for large mammals carfentanil is innocuous, well the reality is that it could be lethal for them too even in small doses, albeit in a smaller percentage of cases compared to humans. As a result of the study, carfentanil is no longer used on moose, but is still mixed with xylazine on some animals who are less sensitive to minute amounts of the opioid.

Opioids can act on several opiate receptors in the central nervous system. To date there are three known varieties of such receptors known as μ , κ and σ. Carfentanil is a strong agonist specific to a pair of μ type receptors known as μ1 and μ2, leading to, among other things, intense euphoria. Interacting with μ1 also leads to analgesic effects, whereas bonding to μ2 leads to respiratory depression, which is the cause of death when the drug kills.

image009
The morphine rule consists of structural criteria for a molecule to have morphine-like effects. The rule applies to natural opiates but no longer to all opioids.

Carfentanil is more potent than fentanyl, which is in turn is far stronger than morphine. Fentanyl was first synthesized from demerol(pethidine)-analogues in the 1960s. Until that point pharmacologists were under the false impression that to act like morphine a drug had to have a quaternary carbon linked to an aromatic ring and to have that quaternary carbon linked to a tertiary amine via a pair of CH2 groups. But fentanyl has only a tertiary carbon, and it is not directly bonded to an aromatic group. It became the first drug to violate the so-called morphine rule, and yet ironically it was stronger than most of the opioids that stuck to the rule.

fentanyl
A comparison of the structures of the two opioids. Carfentanil has an ester group on a carbon that is quaternary and not tertiary. (See green, circled carbon in each case.) Like all opiates both have a carbon linked to tertiary amine (asterisk) separated by a pair of Ch2 groups(green dots). Carfentanil violates the morphine rule by not having its quaternary carbon directly linked to an aromatic ring.

In contrast, carfentanil is like the traditional opiates in that it has a quaternary carbon, one created by the addition of a methyl ester group on that carbon—the only difference in structure between the two molecules. But like fentanyl, it still violates the morphine rule because the carbon is not directly linked to an aromatic ring.

Sources:

Unexpected “gas” casualties in Moscow: A medical toxicology perspective  Annals of emergency medicine May 2003Volume 41, Issue 5, Pages 700–705
http://www.annemergmed.com/article/S0196-0644(03)00109-4/abstract

Unintentional opioid overdose deaths in New York City, 2005–2010: A place-based approach to reduce risk Original International Journal of Drug Policy 25 (2014) 569–574 reveals that carfentanil was a factor in unintentional drug overdose-deaths in a study involving such cases in 2010 to 2014.

RELATIVE EFFICIENCY OF SUCCINYLCHOLINE, XYLAZINE, AND  CARFENTANIL/XYLAZINE MIXTURES TO IMMOBILIZE FREE-RANGING MOOSE   Journal of Wildlife Diseases, 35(1):38-48 http://www.bioone.org/doi/pdf/10.7589/0090-3558-35.1.38
Restraint and Handling of Wild and Domestic Animals 2008 By Murray E. Fowler

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