live, love, hurt

Pretty much everyone knows what acetaminophen is. If you don’t, acetaminophen is the active ingredient in the brand names Panadol and Tylenol. Acetaminophen is known by different names–especially outside the United States–and is most commonly called paracetamol and often abbreviated APAP (from here on out). All of these names come from the chemical name, n-acetyl-para-aminophenol. APAP is notable as one of the first non-opioid (non-narcotic) analgesics without anti-inflammatory properties (this honor actually goes to the drug phenacetin, which was widely used but taken off the market in 1983 due to carcinogenicity concerns. APAP is a metabolite of phenacetin). It is a pain-relieving (analgesic), fever-reducing (antipyretic) drug in the aniline class, of which itself is the only remaining member.

Until recently, pharmacologists did not fully understand APAP’s mechanism. That is, exactly how does it relieve pain and reduce fevers? Considering the only other non-opioid analgesics consist entirely of the non-steriodal anti-inflammatory drugs (NSAIDs) such as ibuprofen and naproxen, APAP’s mechanism was assumed to be a similar one. NSAIDs work by inhibiting enzymes called cyclooxygenase (COX) which produce chemical messengers called prostiglandins which set off inflammation and pain. While NSAIDs markedly reduce inflammation, there is almost little to no inflammation reduction with APAP usage. Why is this?

There are two varieties of cyclooxygenase: COX-1 and COX-2. Most NSAIDs inhibit both of these types equally. COX-1 inhibition has the unwanted side-effect of reducing protective liners in the stomach which can lead to gastric bleeding (indeed, the number one problem with NSAID use). However, inhibition of COX-2 does not produce this effect. Due to this, a number of drugs were developed that selectively inhibit COX-2 while leaving COX-1 alone, and these drugs were called “COX-2 inhibitors” with drug name suffixes of “coxib,” for “COX inhibitor.” Examples of such drugs include valdecoxib, rofecoxib, and celecoxib. A number of these drugs were developed and were very well-regarded by pain management physicians and chronic pain patients alike for their excellent ability to lower pain and inflammation without marked side-effects and even alleviated the need for opioid use (or at least reduced it). Unfortunately some of these drugs were abruptly removed from the United States market and, aside from celecoxib, no new COX-2 inhibitors have been approved or remain on the US market.

So where am I going with this? As APAP’s mechanism becomes more clear, recent findings have suggested that APAP is strongly selective of COX-2 (so much for the need to remove them from the market). So while APAP does indeed inhibit COX like the NSAIDs, there is strong evidence that APAP works through at least two pathways. The first one is well-researched and well-understood (COX inhibition), and the second pathway is what we’re interested in. So what exactly is going on here?

Recent research suggests that APAP may earn its analgesic and antipyretic properties by indirectly activating the endogenous cannabinoid system. The same way that opioids activate our own natural pain-relief system that endorphins and other natural ligands use, the body also has a natural cannaboinoid system which is responsible for the effects of tetrahydrocannabinol, or THC, which is the main active ingredient found in marijuana. Just like morphine binds to opioid receptors (mu, kappa, delta, and others), drugs like marijuana bind to the cannabinoid receptors CB1 and CB2. A well-known natural opioid is endorphin. There are also natural cannabinoids, and the one floating around our brains is called anandamide. The entire purpose of the endogenous cannabinoid system has yet to be fully elucidated, but we will explore some of the regulatory functions they serve below.

When you take APAP, it is metabolized by the body into a number of different chemicals. Some are active, some are inactive. One particular metabolite is taken in by an enzyme in the body called fatty acid amide hydrolase (or FAAH), which converts it into a metabolite called AM404. AM404 is versatile. It’s effect is as an analgesic and an antipyretic (sound familiar?). AM404 inhibits FAAH, which also metabolizes anandamide (the natural cannabinoid). The net effect is that anandamide uptake is inhibited, and levels of anandamide in the brain increase. AM404 also directly inhibits the formation of COX-1, COX-2, and prostaglandins (sound even more familiar?). AM404 also activates a receptor called TRPV1, which is also where the substance capsaicin (the substance that makes hot peppers hot) binds. TRPV1 is responsible for pain transmission and thermoregulatory actions. When activated, TRPV1 enhances and modulates pain transmission, and also tells the body to cool itself down. However, when TRPV1 is bound to for long periods of time it “shuts down,” preventing it from functioning, thus reducing pain.

So let’s take a step back. We’ve got a lot of things going on. Thanks to AM404–which is introduced by acetaminophen–we have the following things going on:

1. AM404 inhibits FAAH–which metabolizes anandamide–resulting in an increase of anadamide.
2. Anadamide binds to cannabinoid CB1 and CB2, and also activates the TRPV1 receptor. Each of these actions are known to inhibit pain on their own.
3. AM404 also activates the TRPV1 receptor.
4. AM404 also inhibits cyclooxygenase and prostagladins.

All of these processes are working to reduce pain (and fever). So, what does this really matter? By investigating these processes we can create novel analgesic drugs that aim to inhibit FAAH in the same way AM404 does (APAP’s use itself is limited due to its toxicity at higher doses) and giving rise to this exact process. We can also make drugs to target TRPV1, and in fact there are already several in advanced testing phases (both agonists and antagonists are being explored, but I’d personally be interested in a partial agonist–can we activate and overload it without causing the burning sensations?).

Let’s remember, this started by looking closely at the metabolism and mechanism of a drug almost everyone worldwide knows of and has made use of: acetaminophen. First we found out that APAP is most likely a highly selective COX-2 inhibitor, and so that trash about taking Bextra and Vioxx off the market was just that: trash. More importantly–if you’ve managed to follow along–you’ve almost certainly deduced that because acetaminophen introduces AM404, and AM404 causes activations in the endocannabinoid system, and in this fashion acetaminophen acts as a pro-drug for a cannabimimetic metabolite (AM404 itself), this means that Tylenol and Panadol and hugely popular drugs containing acetaminophen are activating the endocannabinoid system–like marijuana–in order to produce it’s primary effect of analgesia. Tylenol’s pain-relieving action involves activation of the endogenous cannabinoid system.

And marijuana is illegal?

I’ve written previously on the many side effects of having chronic pain. You have physical and psychological side effects from the medications themselves, and physical and psychological side effects just from the pain itself. The former link is about the physical medication side effects, and the latter is about the psychological pain side effects. This piece is going to concentrate on yet more physical medication side effects, but just for a moment I’d like to expand on the psychological effects from just being in pain.

According to various medical texts, the brain is not designed to be in pain for a long period of time. In fact, the nervous system in general is not designed to send “pain signals” for very long at all. As such, when chronic pain happens often times the neurons that send these signals “learn” to get better at sending them, in the way that your brain learns for memories. A common myth is that people exposed to pain frequently have a higher “pain tolerance,” when in fact the opposite is true. The more often you’re exposed to pain, the more adept your nervous system becomes at sending pain signals. If the same neurons send the same signals for long enough, something called neuronal plasticity happens: in laymen’s terms, those neurons physically change themselves to permanently transmit pain. There is no definitive way to tell when something like this has happened to someone, but it’s a good bet that when all interventional procedures such as disconnecting the nerves themselves (rhizotomy) still fail to provide pain relief, you have this situation. This is bad. There is no way to reverse this process. Your body is now in constant pain. In medical terminology:

Under persistent activation nociceptive transmission to the dorsal horn may induce a wind up phenomenon. This induces pathological changes that lower the threshold for pain signals to be transmitted. In addition it may generate nonnociceptive nerve fibers to respond to pain signals. Nonnociceptive nerve fibers may also be able to generate and transmit pain signals. In chronic pain this process is difficult to reverse or eradicate once established.

Being in constant pain is not only–you know, painful–but it also wreaks havoc on your body. People with high-intensity chronic pain have significantly reduced ability to perform attention-demanding tasks. Pain appears to strongly capture the attention of people with chronic pain; tests assessing the ability to attend show poorer performance than pain-free people on all tests demanding attention. The exception is found with tasks that are highly demanding of attention, where performance between the two groups is equivalent. In experimental testing, two-thirds of individuals with chronic pain demonstrate clinically significant impairment of attention independent of age, education, medication and sleep disruption. Individuals with the highest levels of pain showed greatest disruption of memory traces, suggesting that pain diminishes working memory.

Now that I’ve gotten some of that out of the way, I have another story about physical medication side effects. Last week I was at work. I work some two hours from my home. Right before leaving, I went to use the bathroom but found it difficult to urinate. I didn’t really give this much thought because I’ve had a “shy bladder” for close to a decade. It’s always been stressful and difficult for me to give urine samples for job applications or other reasons. I left work and when I got home a few hours later, I was still unable to urinate. This worried me. Since I usually get home late I tried a few more times and went to bed. When I got up in the morning and got to work I found myself still unable to urinate. At this point I was still leaning toward a shy bladder and figured that if I drank enough I could sort of force it out. This is the technique I use to give urine samples. A few hours later I was very much surprised to find that this didn’t work.

After consuming a large amount of fluids I felt as if my bladder were going to burst and I was in quite a large amount of pain and discomfort. I immediately ran to the bathroom where I was still totally unable to urinate. I realized at this point I had an emergency condition on my hands and tried to convince my carpool to start heading home immediately. By the time they had gathered their things I realized there was no way I was going to make it two hours back home, and decided to run to the hospital across the street from work.

Upon walking into the ER I told the triage nurse I hadn’t urinated for 22 hours and I was in severe discomfort. After a quick sign in with a list of medications and allergies I was rushed off to a room where she quickly (and not as horrifyingly painfully as last time) inserted a Foley catheter where she promptly drained close to 950mL of urine (a normal liquor bottle is 750mL). I felt much better. After a discussion with the physician about the possible causes he was leaning toward my pain medication, as opioids are a frequent cause (but individually rare (less than 10%) side effect) of urinary retention. After a urine analysis (I didn’t have trouble giving a sample this time–it came out of a tube), blood work, and a couple of non-contrast abdominal and pelvic CT scans all came back normal I was told the most likely culprit was the “vast amount of narcotic medications” and advised it would be best the catheter stayed in a few days. I was not entirely happy with that verdict. As I limped out of the ER (and when you’re a male with a tube up your penis, you limp) I was picked up by my carpool and taken home. After a few days of being told my PCP wouldn’t take out a catheter in the office and to go to the ER I realized I could either spend five hours at the bottom of the ER triage or I could take it out myself. I cut the injection port and let the balloon deflate and gently removed it without issue. Within a few hours (and since) I have been urinating just fine.

This story is obviously personal and represents a few things. The physician determined my situation was a side effect of my pain medication and so informed me in such a way as to say “maybe you shouldn’t be taking those medications.” Well, doc, I’d love to not take these medications but without them I can’t function due to Intractable Pain. Maybe if I was on Disability or some other form of I-don’t-work income I could try it and see what happens, but I have a job and a family to support with that job. So firstly it represents bias against people on pain medications, once again. This is a psychological (or maybe even social) side effect of pain medications. Friends and family may shy away from you because you’re on “narcotics” or worse, assume you’re an addict because you’re dependent. I’ve written extensively on how these differ, but the laypeople just fail to understand. There have even been episodes of Intervention-style shows on television wherein a chronic pain patient was accused by a weeping family of abusing their medications and being an addict. A lot of times on these shows it’s true: they’re quite obviously addicts; however, more than once I’ve seen a pain patient taking their medications as prescribed and enduring their side effects only to have this taken as “addiction.” What’s worse is that I’ve seen these legitimate patients forced into rehab more than once because of a bias against medication.

We are sick people. We are sick people with a serious disease. We have pain that’s caused by a disease or is a disease in and of itself. We take medication for that disease, and we withstand the side effects of both that medication and that disease. My only problem is that some of those side effects are man-made, and in a reasonable and educated world should be put to rest. Educate yourselves.