Junk Food, Pt. 2: Other Natural and Synthetic Opioid Peptides

Figure 1. β-endorphin (31 amino acids) structure. Cartoon backbone structure (AF_AFP01203F1) showing the predicted alpha-helical structure. 

In the previous Junk Food Part 1 blog, we explored the world of the exorphins; opioid peptides that form through the enzymatic digestion of dietary proteins. Due to concerns as to the length of that post, I withheld discussion on several topics with the intention of covering these in a second part. And so here we are with Part 2! let’s start by looking into another class of naturally occurring opioid peptides, the endorphins. 

The reason that exorphins or opioid drugs have any physiological effects at all is because we have an endogenous opioid system. This system includes protein opioid receptors and corresponding signaling molecules that activate these receptors. It is these peptide signaling molecules which are called the endorphins. The premier endorphin is β-endorphin. β-endorphin is the typical endorphin that scientists or a journalist refer to when talking about endorphins mediating a runner’s high or any other endorphin mediated hedonic experience [Janal et al. 1984, Schwarz et al. 1992]. The exorphins encountered in the first part of this blog post, are rather small peptides, 3-8 amino acids in size. In contrast, β-endorphin (Figure 1) is a beast of a peptide containing 31 amino acids. β-endorphin can adopt an alpha-helical secondary structure, which is shown in Figure 1. Like exorphins, endorphins originate, through enzymatic hydrolysis of precursor proteins. β-endorphin, in particular, forms from the enzymatic cleavage of a protein called proopiomelanocortin (POMC). In addition to β-endorphin, metabolism of POMC gives rise to several other physiologically relevant peptides including adrenocorticotropic hormone (ACTH), melanocyte stimulating hormones (MSHs) and several other “endorphins” including β-endorphin(26), β-endorphin(1-27), α-endorphin (16 amino acids) and γ-endorphin (17 amino acids).  Other physiologically important endorphins include the enkephalins, dynorphins, nociceptin, and endomorphin-1 and endomorphin-2 [Hackler et al. 1997, Zadina et al. 1997, Papini et al. 2011, Margolis et al. 2023]. 

As stated above, the endogenous opioid peptides act as signaling molecules (i.e., neurotransmitters and hormones). That is to say they binding to and then activate one or more of the opioid receptors: MOR, KOR, DOR and NOP. All four opioid receptors are G protein-coupled receptors (GPCRs), receptor proteins which are typically found in the cell membrane and once activated signal by activating intracellular proteins called G proteins. These go on to create their own signals which leads to various changes to the cell. This is how the binding of a molecule and resulting receptor activation impacts a cell. The pharmacological importance of GPCRs is exemplified by the fact that genes for GPCRs make up around 4% of our entire gene-coding genome and approximately one third of all Federal Drug Administration (FDA) approved drugs act by targeting GPCRs. 

The varieties, receptor preferences and precursor proteins of key endorphins are summarized in Table 1 To quickly summarize, β-endorphin and the endomorphins are typically considered fairly selective MOR agonists, i.e., activators of MOR [Koneru et al. 2009, Monory et al. 2000]. The physiological relevance and opioid activity of the smaller β-endorphin fragments mention above are still being sorted out [Chaudhry and Grossman. 2017, Gomes et al. 2020, Margolis et al. 2023]. Enkephalins are largely DOR agonists, whereas the dynorphins are KOR agonists [Koneru et al. 2009]. Finally, we have the lone peptide nociceptin, which is an agonist at NOP, the most recent opioid receptor discovered [Koneru et al. 2009]. More recent research suggests that the receptor selectivity of the endorphins shown in Table 1, is not as clean as previously thought, and there is evidence that many endorphins are promiscuous, capable of binding to multiple opioid receptor partners. The extent to which this occurs in vivo is unclear, and the mingling habits of the endorphins and opioid receptors is an active area of study [Gomes et al. 2020, Margolis et al. 2023]. 

 The term “opioid” is colloquially used to describe compounds that act as agonists at the MOR. This is because MOR is the opioid receptor that mediates the characteristic effects of infamous opioids like morphine, oxycodone and fentanyl. The MOR is actually named after morphine [McDonald et al. 2005]. The natural product, morphine was the first opioid molecule discovered and the very first alkaloid isolated by humans. The non-MOR opioids like KOR agonists – salvinorin A and niravoline or DOR agonists DPI-22 and BU-48. While some of you may have heard of salvinorin A these are definitely not household names. Thus, the term opioid has come to imply MOR opioids and when referring to opioids that target other receptor subtypes, we generally specify “kappa opioids” or “delta opioids” etc. As in the first blog we will focus mostly on MOR here. 

The endogenous MOR system regulates various physiological processes in humans, and other mammals, including pain processing, emotional states, hedonic reactions in reward processing, smooth muscle function, gastrointestinal activity, itch, endocrine function, immune regulation, and much more [Le Merrer et al. 2009, Ninković and Roy. 2013, Galligan and Akbarali. 2014, Jaschke et al. 2021]. When you consume a MOR agonist like morphine, it mimics endorphins and stimulates MORs. This stimulation leads to the physiological and subjective effects including pupil constriction, constipation, analgesia, and euphoria. Although a single gene encodes the MOR, called OPRM1, when it is expressed, it can produce distinct protein variants of MOR through a process called alternate splicing. These MOR “slice variants” have distinct physiological properties and can be found in different areas of the body, an organ, on different cells, etc [Liu et al. 2021]. We are only beginning to understand the significance of this phenomena. 

Unlike the exorphins and endorphins, conventional opioid medications like morphine, oxycodone, and fentanyl are not peptides, but rather “small molecules”.[1] These small molecules bind to the same binding site on MOR as the peptides do. This binding site, which is found in a similar location within the extracellular portion of the helical bundle of most class-A GPCRs, is called the orthosteric binding site. Molecules effect receptors by forming bonding interactions with the amino acids of the protein, pushing and pulling and altering the conformation or shape of the protein. Although they are not peptides, it is notable that the opioid alkaloids morphine and codeine are biosynthetically derived from the amino acid tyrosine [Kirby. 1967]. In fact, many alkaloids and biomolecules originate from amino acids[2].  Mescaline, the psychedelic alkaloid in various cacti species, is also derived from tyrosine and the psychedelic N,N-dimethyltryptamine (DMT) is derived from tryptophan. Many of our neurotransmitters (e.g., serotonin, dopamine, norepinephrine, histamine, and GABA) are too, and in some cases, nature seems to have gotten indolent and decided to just go ahead and use amino acids directly as neurotransmitters (e.g., glycine and glutamate). 

Figure 2. Peripherally-selective tetrapeptide morphiceptin and pentapeptide casokefamide.

Now that we have covered the endorphins and our endogenous opioid system, let’s take a look at some human-made or synthetic opioid peptides. Morphiceptin or NH2-Tyr-Pro-Phe-Pro-CONH2 (Figure 2), is a derivative of, meaning it can be prepared from, the exorphin β-casomorphin-4 (BCM-4). In fact, the structure of morphiceptin is BCM-4, with the C terminus carboxylic acid group (COOH) replaced by an amide (CONH2). This carboxylic acid to amide switch is a common modification in synthetic peptides, and occurs in some “natural” peptides.  The reason is that it can enhance metabolic stability and receptor pharmacology (e.g., improve binding, functional activity, etc). Morphiceptin is a MOR agonist with good selectivity over DOR [Chang et al. 1981]. This selectivity allowed morphiceptin to be used as a valuable tool in early opioid receptor research, helping construct our understanding of the different opioid receptors. This illustrates how molecules can serve as tools that help us build our models of reality. In an unexpected twist, it turns out morphiceptin may not be synthetic after all. A few years after being synthesized by humans, morphiceptin was reportedly found in an enzyme digest of milk proteins, suggesting it may be an exorphin [Chang et al. 1985]. This would not be the first time something like this happened. However, while exciting, it is important to point out that I have not seen this replicated, and mistakes happen in science all the time. 

Figure 3. Amino Acid Structure and Chirality.

Another synthetic peptide, is casokefamide (Figure 2), also known as NH2-Tyr-D-Ala-Phe-D-Ala-Tyr- CONH2 and  β-CM-4027, which also takes its inspiration from an exorphin. In this case it is β-casomorphin-5 (BCM-5) [Brantl et al. 1982]. The structure of casokefamide contains two key modifications that enhance its metabolic stability; a C-terminus amide and inclusion of two D-amino acids, in place of the "standard issue” L-amino acids. With respect to L and D-amino acids: 19 of the 20 amino acids common in biology have a property called “chirality”.  In a simple sense this means that they can exist in either of two forms, a D- or L- isomer form. This has to do with how the structure projects in 3-dimensional space (Figure 3). More specifically it is the way the substituents project out from the chiral alpha carbon (Figure 3). In the example in Figure 3, the H is implicit and not shown, thus there are 4 different groups coming off the alpha carbon shown in alanine, making it chiral. Two isomers are possible, a D- and L- isomers. These are called enantiomers and are non-superimposable mirror images of each other. Take a moment and try to superimpose your right and left hands, most readers will quickly find this is not possible. This is a good everyday analog of this property of non-superimposable mirror images. If by some chance you are able to superimpose your hands, please contact me immediately! After trying it with your hands, try this with the structures of L- and D- alanine shown in Figure 3, note the broken-line wedge indicates that that bond projects into the screen or page, and thus away from you. A solid dash would indicate the bond projects towards you. For reasons we do not yet understand, the biochemistry on earth loves L-amino acids, to the point of obsession. D- amino acids are extremely rare in “natural” peptides or proteins. The significance of this is that proteins are chiral, which is typically why only one isomer of a chiral drug will display the desired activity at a receptor. When present in a peptide, a D-amino acid can thus confer metabolic stability, as enzymes will struggle to recognize them, as they expect to find L-amino acids. The rationale for including the terminus amide and the D-amino acids in casokefamide was to increase DOR activity and improve metabolic stability. The result is a peripherally selective balanced MOR and DOR agonist [Brantl et al. 1982, 1993, Hautefeuille et al. 1986]. One potential application for casokefamide is as an antidiarrheal. In a single open-label clinical trial, casokefamide was tested in 10 healthy males. At oral doses of 5.5, 8 and 16 mgs, “a trend toward prolongation of oro-caecal transit time” was observed, relative to placebo. Aside from some flatulence, casokefamide was “well tolerated” [Schulte-Frohlinde et al. 2000]. Despite the “trend”, it appears no subsequent clinical investigations or development occurred. An important thing to be aware of is that the term “trend” is often used in science as a fluffing term which roughly translates to: we found a small effect if we look hard enough, but it is not statistically significant.

Earlier, I had mentioned that casokefamide was a peripherally selective opioid, meaning it will act selectively in the body outside the central nervous system (CNS), which is comprised of the brain and spinal cord. You may be wondering what use is there for a peripherally selective opioid? Why not just take morphine for diarrhea, as humans have done for thousands of years? It is almost a certainty that when your Great-Great-Great grandmother had an attack of diarrhea, it was morphine that provided relief. The issue with this is that the liberal use of opioids medically in the past often led to substance use disorders, particularly as people were rarely informed of the risk or knew that they were even consuming opioids. Loperamide, the active ingredient in Imodium®, is a peripherally selective synthetic MOR agonist developed by Janssen Pharmaceuticals. Loperamide is a highly effective and well-tolerated antidiarrheal agent that is included in the World Health Organization (WHO) Essential Medicines List. With peripheral selectivity, loperamide works as well as morphine for diarrhea but lacks the centrally-mediated psychoactive effects.[3]  Devoid of these psychoactive effects, loperamide can be sold over-the-counter, even in the highly chemophobic United States. Notably loperamide is a small molecule and the reason for its peripheral selectivity (Pgp transporter mediated efflux) is different from that of the peptides (polarity and size) we are talking about. But the clinical significance of the peripheral selectivity is the same.  

Figure 4. Tetrapeptide frakefamide containing a synthetic amino acid 4-fluorophenylalanine.

The next synthetic opioid tetrapeptide I want to discuss is NH2-Tyr-D-Ala-(p-F)Phe-Phe- CONH2 or frakefamide (Figure 4). Sadly, I have not found details on the origin of frakefamide. It is, however, structurally similar to the enkephalins, as well as to a synthetic enkephalin inspired molecule, we will discuss next called DAMGO. The structure of frakefamide is remarkable as in addition to a D-amino acid and a C-terminus amide, it contains an “artificial” amino acid known as 4-fluorophenylalanine. Although organofluorines are extremely rare in biochemistry, fluorine has quickly become essential in drug design, conferring many desirable pharmacological properties [Sun and Adejare. 2006, Gillis et al. 2015, Meanwell et al. 2014]. As with morphiceptin and casokefamide, frakefamide is a peripherally selective MOR agonist. Surprisingly and despite this, preclinical animal studies suggested frakefamide has analgesic effects through its actions on peripheral MORs. For this reason, frakefamide reached clinical development as an analgesic by AstraZeneca and Shire. At least two reports on its analgesic action in humans exist from clinical trials, published as abstract conference proceedings from the 10th World Congress on Pain [Jacobsson et al. 2002, Becktor et al. 2002]. Unfortunately, I am currently unable to find the full abstracts, though the titles clearly emphasize analgesic activity, as do subsequent publications citing the abstracts. Notwithstanding the apparently promising clinical efficacy data, there seem to be no subsequent studies on analgesic effects and it appears that development for this purpose was abandoned. Were the benefits reported in the abstracts just a “trend”? I don’t know. At least one later small clinical study in 12 healthy males exists that assessed the ability of frakefamide to induce respiratory depression, a sign of central MOR agonism. Frakefamide did not suppress respiration in this study, supporting its peripheral selectivity at the dosage used (1.22 mg/kg).  Nausea, itchiness and muscle pain following injection was reported by volunteers [Modalen et al. 2005, 2006].

The concept of achieving analgesia with a peripheral MOR agonist is exceptionally powerful. Such molecules would be essentially devoid of the abuse liability characteristic of centrally active MOR agonists. In addition to frakefamide, several other peripherally selective opioids, including loperamide, have been evaluated as analgesics in rodent and small human trials [Schmidt. 2003, Tegeder et al. 2003, Martínez and Abalo. 2020]. The results are mixed and too complex to review here. While there has been promise in preclinical or small clinical studies, we cannot escape the unfortunate reality that no peripherally selective MOR agonist is approved as an analgesic drug. As far as I can tell, none are currently in or have been evaluated in a phase III clinical trial. Phase III trials assess therapeutic efficacy and safety, in a large group of patients, and are a requirement for regulatory approval in most countries. While the development of loperamide for diarrhea is a significant medical achievement, if effective, a peripherally selective MOR analgesic, would represent an unparalleled innovation in pain management. Such a drug would be absolute blockbuster, grossing billions in profit annually. I point this out as there is clearly incentive for capitalists to discover and develop these compounds. In fact, medicinal chemists and pharmacologists have been searching for an opioid analgesic devoid of abuse liability for well over a century with little success. Many approaches, including peripheral selective MOR agonists, receptor selectivity, pharmacokinetics have been pursued. I suspect the reason for the absence of such agents in the pharmacopeia, is that the central MOR population, especially those rich in limbic regions of the brain, are indispensable to the powerful analgesic action of opioids. Unfortunately, it is this same population that appears to underlie the euphoria and abuse liability. I personally think separating the euphoria from the analgesia will be extremely difficult. The reason being is that my hunch is the euphoria is not just a MOR “side effect”, but rather is a key contributor to the “analgesia”. I am not alone in believing that opioids do not so much eliminate pain like a local anesthetic would, as much as they make it less distressing and bothersome [Kimmey et al. 2022]. Neuropharmacologist and opioid expert, Dr. Grégory Scherrer addresses this by pointing out: “patients who take opioids for pain report that they can still feel the sensation of pain but say it’s less bothersome — the emotions of pain are different” [Armitage. 2019]. When you are wrapped in that warm MOR-stitched blanket of contentment, the awareness and emotional impact of pain diminishes. To be clear, I am not asserting that targeting peripheral opioid receptors for analgesia is hopeless or that the euphoria is the sole driver of opioid analgesia. I am well aware of evidence for MOR-mediated suppression of spinal pain signals and the gate control theory of pain and believe these could play a role [Mendell. 2014]. However, devoid the euphoria and related psychoactive effects, I suspect any resulting MOR agonists, central or peripheral, will lack the same analgesic wallop of the classic MOR analgesics. 

Figure 5. Structure of the synthetic opioid tetrapeptide DAMGO.

The synthetic tetrapeptide NH2-Tyr-D-Ala-Gly-MePhe-NH(CH2)2OH or DAMGO (Figure 5) is truly indispensable. First described in 1981 by Handa et al., DAMGO was designed as a stable analog of the DOR-preferring agonist Met-enkephalin. The etymology of the name DAMGO is an acronym that alludes to its structure: D-Ala, Methionine-Enkephalin-Glycolamide. DAMGO sports several notable modifications including a D-alanine, an N-methyl amide, and a C-terminus ethanolamide substitution. Despite the enkephalin lineage, this apple fell far from the tree with respect to its pharmacological profile, as DAMGO is a potent and selective MOR agonist. This type of pharmacological change is not uncommon; even small changes to a molecular structure can alter the pharmacology. If you are involved in MOR research you have at least heard of DAMGO and probably have used it. A quick Google Scholar search of “DAMGO” I just did gave 17,000 results. When my lab recently did some MOR dependent G protein dissociation bioluminescence resonance energy transfer (BRET) signaling studies we used DAMGO as a full agonist reference compound. The binding structure of DAMGO at the MOR was recently solved using cryo-electron microscopy [Zhuang et al. 2022]. A snapshot of DAMGO in the orthosteric binding site of MOR based on this work is provided in Figure 6. Like the other peptides discussed, DAMGO poorly penetrates the BBB. However, it seems that with a large enough dose brute force wins with central opioid activity being achieved with high doses given by subcutaneous (SC) or intravenous (IV) injection in rodents [Van Dorpe et al. 2010, Lindqvist et al. 2016a]. I’m not aware of any reports or studies on humans being administered DAMGO. In addition to opioid research, many scientists use DAMGO as a model peptide to test new pharmaceutical formulation strategies that improve peptide brain penetration [Lindqvist et al. 2013, Lindqvist et al. 2016b]. The reason DAMGO is used in these studies is because its pharmacology is well understood and it is accessibility. This again illustrates how molecules can serve as powerful research tools. 

Figure 6. DAMGO (grey backbone) bound to the orthosteric binding site of MOR. Only DAMGO, the protein alpha-helical backbone, and key amino acid side chains involved in binding are displayed. PDB: 8EFQ. 

Figure 7. Structure of the Synthetic Pentapeptide Metkefamide.

So far, the opioid peptides we've discussed all demonstrate poor BBB penetration. You may be wondering, is this common to all peptides? Although it is fairly typical with peptides, some peptides efficiently cross the BBB. One example is the synthetic opioid peptide NH2-Tyr-D-Ala-Gly-Phe-(N-Me)-Met-CONH2 or metkefamide (Figure 7). Metkefamide is also called metkephamid and LY-127,623, indicating it's Eli Lilly origin. Metkefamide is a pentapeptide, and similar to DAMGO the structure was based on Met-enkephalin [Frederickson et al. 1981]. The metkefamide structure combines several of the modifications we’ve seen: a C-terminus amide, a D-Alanine amino acid, and an amide N-methylation. Metkefamide is reported to be about equipotent as an agonist at MOR and DOR with some affinity for KOR and NOP as well. Just FYI NOP was initially thought to be a KOR subtype called κ3 [Frederickson et al. 1981, Burkhardt et al. 1982, Clark et al. 1989]. DOR agonism is believed to mediate at least some of metkefamide’s analgesic action [Hynes and Frederickson. 1982]. A study in 4 male volunteers found that metkefamide suppresses plasma vasopressin, which is supportive of the fact that it can cross the BBB and bind to central MOR opioid receptors in the hypothalamus [Zerbe et al. 1982]. In a randomized double blind study of 59 women experiencing severe postpartum episiotomy pain, IM metkefamide (70 and 140 mg) was evaluated against placebo and an active comparator, MOR agonist meperidine (100 mg). 140 mg metkefamide “was the most effective”, followed by meperidine (100 mg), and both outperformed placebo outperformed placebo [Calimlim et al. 1982, Bloomfield et al. 1983]. In this study patient reported side effects include fatigue, dry mouth, eye redness, burning at the injection site, sensation of heaviness of the extremities, and nasal congestion [Calimlim et al. 1982]. Interestingly, in the Zerbe et al. (1982) study patients also reported “a subjective sensation of heaviness of the extremities”, eye redness, nasal stuffiness and dry mouth. I’ve found no follow up studies evaluating its use as an analgesic, though I wonder if the side effects were a factor?  

Figure 8. Heptapeptide Dermorphin. Isolated from skin secretions of frogs in the genus Phyllomedusa. 

The final opioid peptide I want to discuss with you is dermorphin (Figure 8) or NH2-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-CONH2, a naturally occurring heptapeptide found in the skin secretions of South American frogs in the genus Phyllomedusa, most notably the giant monkey frog or Phyllomedusa bicolor. Indigenous peoples of the Amazon basin traditionally use dermorphin-rich secretions from Phyllomedusa bicolor as a folk medicine known as “Kambo” or “Sapo”. Dermorphin is extraordinarily potent and selective MOR binder, with picomolar binding affinity, which is about as high as it gets with reversible binding. It is also a MOR agonist [Negri et al. 1992, 1995, Smith et al. 2022]. Although it is a natural peptide, dermorphin notably contains two features we saw in the synthetic peptides: a D-amino acid and a C-terminus amide group [Montecucchi et al. 1981, Hesselink and Schatman. 2018]. It seems nature mastered these tricks long before humans began intentionally employing them! Dermorphin is significantly more potent than morphine in vitro and in vivo, and exhibits MOR agonist-mediated effects on locomotor activity and analgesia following IV injection in mice and SC and intracerebroventricular (ICV) injections in rats [Broccardo et al. 1981, Puglisi-Allegra et al. 1982, Negri et al. 1995]. One promising study in rats found that tolerance and physical dependence was “less marked” with dermorphin compared to morphine [Broccardo et al. 1981].  The reported IV and SC activities, suggest dermorphin crosses the BBB. As to this possibility, studies are mixed, but overall, it does appear dermorphin can induce central effects at relatively high doses [Hesselink and Schatman. 2018, Van Dorpe et al. 2010]. This seems to be the case with DAMGO as well and possibly the other opioid peptides we’ve discussed. Unfortunately, no studies in humans have tested if peripherally administered dermorphin has central effects. However, dermorphin (20 μg) was administered to postoperative pain patients who underwent elective surgery by intrathecal injection (direct injection into the cerebral spinal fluid) in a randomized double blind trial (N=150 patients). In this case dermorphin had strong analgesic effects and was well tolerated with side effect similar to the other opioid comparators [Basso et al. 1985]. Unfortunately, this seems to be the only human clinical study [Hesselink and Schatman. 2018] and ICV is not exactly a patient friendly route of administration. Ok so what about Kambo? Do the effects reported by people who consume Kambo provide us with any insights into if dermorphin has central MOR effects? That is difficult to answer scientifically. One complexity is that in addition to dermorphin, Kambo contains many other bioactive peptides, each with their own complex pharmacology [Thompson and Williams. 2022]. Kambo contains a number of potent MOR agonists [Broccardo et al. 1981, Negri et al. 1995], as well as various deltorphins (e.g., deltorphin A, deltorphin  I and II) which are potent and selective DOR agonists. Notably the deltorphins are reported to have high brain penetration [Kreil et al. 1989, Fiori et al. 1997]. This complex chemical make-up complicates any simple conclusion as to the contribution of dermorphin to the effects of Kambo. 

Ok, so we don’t know which peptide are doing what, but is the experience central opioid-like effects in any way? This is a good question and should at least tell us some MOR agonist peptide is getting in the brain. Unfortunately, the answer in short is … no. A typical Kambo experience does not resemble an opioid experience and is not for the faint of heart. The features that characterize a typical Kambo response is extreme swelling of the face due to edema, a drop in blood pressure, gut wrenching nausea and vomiting, and profuse sweating. MOR agonists can certainly cause sweating and nausea so maybe these are MOR mediated in part? It's difficult to say without some controlled experiments, like naltrexone pretreatment, that I do not believe have been done. Fortunately, the distressing experience  lasts about 30 minutes and is often followed by a pleasant “afterglow” that includes elevated mood, increased stamina, and a clarity of thoughts that lasts for several days or more [Schmidt et al. 2020]. Maybe the afterglow is MOR mediated, but that is definitely not clear and given the reported timeline of effects this seems unlikely. Another possibility is that you are just thrilled to be alive after such an intense physical jolt. Maybe you the same state of bliss can be obtained by sucking on a downed powerline? Of course, there could actually be a lasting physiological change that underly the sustained mood lift, independent of the acute “bells and whistles”. Sounds a bit like the debate over the role of the psychedelic experience in the therapeutic response, doesn’t it?! Unfortunately, there is little research on the topic with Kambo, so I really don’t know. Certainly, many claim that Kambo has powerful therapeutic benefits and it is widely used, albeit with scant clinical evidence, for many indications including depression, substance use disorders, chronic pain, post-traumatic stress disorder, Lyme disease and more [Labate and Lima. 2014, Thompson and Williams. 2022]. I should point out that while most people quickly recuperate from the acute effects, there have been severe adverse reactions including deaths attributed to Kambo [Kumachev et al. 2018, Aquila et al. 2018, Sacco et al. 2022]. I’m not trying to fear monger here, rather I’ve seen some concerning acute reactions to Kambo in videos published online. As someone who has had allergic reactions, the edema in particular freaks me out. Acute facial edema like this is generally a sign that something is going on with your immune system or possibly your cardiovascular or lymphatic system. One final point, pure dermorphin has been used as a performance enhancing drug in race horses, presumably as a potent analgesic to evade forensic detection. This was the subject of a federal criminal investigation and conviction [Bogdanic. 2012, United States Department of Justice 2017]. 

While nature has found many uses for opioid peptides, it is notable that not a single opioid peptide is an approved drug.[4] This is not for a lack of trying. Despite these failures, which are fairly typical of drug discovery, scientists continue to explore synthetic opioid peptides inspired by exorphins, endorphins, and related natural or synthetic opioid peptides like DAMGO and dermorphin [Smith et al. 2022, Gach-Janczak et al. 2024]. It's not all about drugs, we can use medicinal chemistry to make really useful tool molecules too. And we learn a lot by just exploring and “playing around” with the creative process. DAMGO is an excellent example of that. I hope these efforts continue, and I am sure that the Universe of opioid peptides will continue to grow. 

Footnotes:

[1] The term small molecule is, not surprisingly, based on size of the molecule – or more technically the “molecular weight”. Molecular weight is a measure of how heavy a molecule is using atomic mass units or Daltons. Weight is a relative measurement: a Dalton (Da) is by definition exactly one-twelfth the mass of a carbon-12 atom. 1 Da = 1.66053906660 × 10−27 kg. It thus easier to use units of Daltons than kilograms or grams etc. when dealing with molecules. The typical molecular weight cut-off to be considered a small molecule is  ≤1000 Da. Most peptides fail this criteria and are therefore by definition not small molecules. 

[2] A chemical intermediate in the biosynthesis of morphine is the neurotransmitter dopamine [Ziegler et al. 2009]. Stefano and Kream (2007) have proposed that the evolutionary origins of catecholamine (e.g., dopamine, adrenaline, and noradrenaline) biosynthesis arose from the enzymes involved in morphine biosynthesis. These authors have also suggested that morphine is endogenously produced in mammals including humans [Kream and Stefano. 2006, Mantione et al. 2010].  While I really want this to be true, and have my fingers crossed as I write this, I am skeptical. I suspect environmental contamination of samples,  laboratory contamination, or some type of data misinterpretation are a more likely explanation. An interesting example of a likely case of environmental contamination is the detection of the MOR agonist tramadol in Nauclea latifolia, a Cameroonian medicinal plant. It is thought this resulted from the agricultural use of tramadol as an analgesic for cattle [Kusari et al. 2014, 2016]. However ,there is still some debate on this question [Agostini et al. 2019]. People have reported various psychoactive compounds as being present in plant or animal tissue. I’ve worked in science long enough to know contamination, mix-ups, and mistakes in interpretation happen and am always skeptical until it's been replicated by several labs. At least one study supports endogenous morphine production by a human cell line using traceable radioactive isotopes, and gas chromatography tandem mass spectrometry [Poeaknapo et al. 2004]. 

[3] Opiophiles have made efforts to facilitate loperamide’s entry into the brain. And why not, loperamide would be a potent, accessible, and inexpensive full MOR agonist. Many opioid users also use loperamide to alleviate opioid withdrawal symptoms. It may be that a subset of withdrawal symptoms are mediated by peripheral MORs, or that with high doses a small amount gets into the brain. Originating in the underground, I’ve recently been seeing medical interest in this use. 

[4] Though not a peptide, racecadotril is a small molecule drug which serves to increase endogenous opioid peptides. It is a peripherally active inhibitor of an enzyme called neprilysin (enkephalinase). Neprilysin breaks down enkephalins, and thus when it is inhibited the levels of enkephalins increase. As pointed out in Table 1, the enkephalins act mostly as DOR agonists. Though we have not discussed this, DOR agonism can reduce water secretion in the intestines, decreasing diarrhea without inducing the constipation seen with MOR agonists. RB-101 is a related compound which is a centrally active enkephalinase inhibitor under investigation for various indications including depression, pain and anxiety disorders. I have lied awake in bed many a nights, with a head full of dopamine, fanaticizing about developing a drug that does a similar thing to enhance the levels of MOR agonist endorphins. Unfortunately, it appears the breakdown of β-endorphin and the endomorphins appears to be quite complex, involving multiple enzymes which are further involved in metabolizing various other physiologically important peptides. Thus, achieving this would likely be difficult to selectively target clinically. 

Acknowledgements:

Sam Greaves and Philip White helped with edits and provided feedback. Sam Greaves formatted this and our other blog posts for the website. Chris Orme assisted in generating the cover image using DALL-E by OpenAI and PyMOL. 

References

Agostini, M., Marcourt, L., Taïwe, G.S., Boucherle, B., De Waard, M., Wolfender, J.L., Queiroz, E.F. and Boumendjel, A., 2019. The Tramadol origin: the end of a story or an endless controversy?. Planta Medica, 85(18), pp.P-472.

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