Menstruation Pulls the Trigger

Menstruation pulls the triggerfor inflammation and pain inendometriosisAlexis Laux-Biehlmann1, Thomas d’Hooghe2, and Thomas M. Zollner1

1 Global Drug Discovery, Bayer Pharma AG, Mullerstraße 178, 13353 Berlin, Germany2 Leuven University Fertility Center, University Hospitals, UZ Gasthuisberg, 3000 Leuven, Belgium

Opinion

Glossary

Abnormal uterine bleeding: bleeding from the uterine corpus that is abnormal

in duration, volume, regularity, and/or frequency.

Deep infiltrating endometriosis: infiltration of endometriotic lesion into any

given structure to a depth of at least 5 mm (e.g., uterosacral ligaments, rectum,

Endometriosis is a chronic, hormone-dependent, inflam-matory disease, characterized by the presence andgrowth of endometrial tissue outside the uterine cavity.It affects 5–10% of the female population of reproductiveage and is frequently associated with moderate to se-vere pain, subfertility, and a marked reduction in health-related quality of life. Here, we propose a new patho-physiological concept of endometriosis, summarizingrecent findings in one unifying picture. We proposemenstruating tissue as the trigger for inflammatory painin endometriosis through the activation of innate im-mune cells and peripheral nerve endings. We speculatehow innovative treatment modalities beyond hormonaltreatment will improve patients’ lives.

The burden of endometriosisEndometriosis is defined as the presence of uterine stromaand glands outside the uterine cavity. It is one of the mostfrequent disorders of the female reproductive tract, affectingapproximately 5–10% of women of reproductive age andrepresents a significant disease burden [1–3]. Key symp-toms are: (i) abdominal pain in different forms, such aschronic pelvic pain (CPP), dysmenorrhea (see Glossary),dyspareunia, or dyschezia (Box 1); and (ii) subfertility. Morethan 50% of patients have these symptoms, which impacttheir relationships, their work, and their health-relatedquality of life [1–3]. Bleeding disorders, such as short cyclelength, heavier menstruation, and longer flow duration, arealso frequently observed and known as risk factors [4,5]. En-dometriosis symptoms are also difficult for patients to han-dle because there is an average delay of 6.7 years betweendisease onset and final diagnosis [5,6]. The burden of endo-metriosis has also significant economic consequences withhealthcare expenses and costs due to productivity loss simi-lar to those of other chronic diseases, such as diabetes,Crohn’s disease, or rheumatoid arthritis [3].

The pathophysiology of the disease is incompletely un-derstood. The risk of developing endometriosis is greater inwomen with increased quantity of menstruation due to

0165-6147/

� 2015 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tips.2015.03.004

Corresponding author: Zollner, T.M. ([email protected]).Keywords: retrograde menstruation; inflammatory pain; neurogenic inflammation;sensory nerves; endometriosis.

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abnormal uterine bleeding [7–10]. Furthermore, the biologyof eutopic endometrium is different in women with endome-triosis compared with controls [11,12], which probably is aconsequence of the development of endometriosis accordingto preliminary evidence in baboons [13]. Among the differenthypotheses, Sampson’s theory of retrograde menstruation isthe most widely accepted [14]. During menstruation, men-strual debris travels not only anterogradely to the vagina,but also in a retrograde manner through the fallopian tubeinto the peritoneal cavity [14]. Additional sources of tissuedebris are endometriotic lesions, which have been observedto bleed during menstruation [15,16]. Here, we introduce theterm ‘extra-uterine menstruation’ to combine bleeding endo-metriotic lesions and retrograde transfer of tissue fromuterine menstruation (retrograde menstruation). However,retrograde menstruation occurs in most women, while only aminority develops the disease. This implies additional caus-ative factors, such as genetic susceptibility, autoimmunity,or anomalies in the inflammatory response [8,17–19]. As anexample, genetic susceptibility factors have been identifiedin genome-wide association studies, although these current-ly cannot fully explain the disease in affected women [20].

Endometriosis has features of a hormone-dependentdisease, because its symptoms are usually restricted tothe reproductive period and are responsive to hormonaltreatment (Figure 1A). Sex hormones might influence thedisease through their proliferative, pro-nociceptive, andproinflammatory effects [1,21,22]. However, beyond hor-monal aspects, extra-uterine menstruation, inflammation,and peripheral nerve endings have a major role in thepathophysiology of the disease (Figure 1B,C). Recentreviews focused on the interaction of immune cells andsensory nerves in the generation of endometriotic pain

rectovaginal septum, vagina, or urinary tract).

Dyschezia: painful defecation.

Dysmenorrhea: pain associated with menstruation.

Dyspareunia: painful sexual intercourse due to a medical condition.

Nociception: neural process of encoding noxious stimuli.

Nociceptive neurons: central or peripheral neurons of the somatosensory

nervous system that are capable of encoding noxious stimuli.

Progestin: natural or synthetic substance having progesterone-like activity.

Box 1. Pain in endometriosis

Endometriosis-associated pain, the main symptom of the disease, is

heterogeneous and complex. The most common symptoms are

dysmenorrhea, dyspareunia, and CPP. Two types of pain perception

system can be involved, depending on the localization of the lesions

in the pelvic cavity. Somatic pain comes from the stimulation of the

sensory nerves present in the skin or deep tissues, such as the parietal

peritoneum [91]. In opposition, pain arising from internal organs,

such as the uterus, bladder, or rectum, is called visceral pain [92]. The

mechanisms underlying the generation of pain are divided in two

main categories, nociceptive and neuropathic pain [93]. It is under

debate which of these is predominant in endometriosis [23]. Impor-

tantly, the involved pathophysiological mechanisms differ and,

consequently, the therapeutic strategies are not the same [93,94]. Neu-

ropathic pain is a pathologic condition caused by a lesion or a disease

of the somatosensory nervous system that leads to an altered neural

processing [95,96]. Nociceptive pain arises from actual or threatened

damage to non-neural tissue and is due to the activation of

nociceptive neurons. When nociceptive pain is associated with

inflammation, it is called inflammatory pain [95,97]. Peripheral

sensitization of nociceptive neurons can occur in both, leading to an

increased responsiveness to noxious (hyperalgesia) and innocuous

(allodynia) stimuli.

Both neuropathic and inflammatory pain are often cited in the

context of endometriosis-associated pain [98]. In contrast to what is

often claimed in the endometriosis field, we believe that neuropathic

pain aspects are not in general representative of the pathophysiology

underlying pain in the disease. Central sensitization is described in

women with endometriosis-associated pain, and this is often used as

an argument for neuropathic pain. In these patients, an increased

excitability of the CNS to normal and/or innocuous stimuli leads to

increased pain hypersensitivity and larger referred pain areas

[98,99]. However, central sensitization can arise in the context of

both neuropathic and inflammatory pain, and occurs in several

inflammatory-driven painful diseases, such as migraine and irritable

bowel syndrome [100]. The neurotrophic properties of endometriotic

lesions are also often used as argument for neuropathic pain in

endometriosis, but they are also strongly related to inflammatory pain

mechanisms, especially through NGF [101,102]. Finally, the positive

effect of the surgical removal of endometriotic lesions on pain

symptoms undermines that neuropathic pain mechanisms are key in

endometriosis [103]. Nevertheless, surgically induced neuropathic

pain occurs in 10–40% of patients following general surgery [104]. It

may also be one of the causes of recurring pain following laparo-

scopic surgery in the absence of detectable lesions [105,106].

To summarize, endometriosis is a sterile inflammatory disease

within the peritoneal cavity and, as described here, there is abundant

evidence suggesting that endometriotic pain originates, to a large

extent, from inflammation.

Pain percep�on

Thalamus

GnRH

Hypothalamus

Anterior pituitary

Ovary

+

+

EstrogenProgesterone

+

–

Uterus

Sensory nerves

FSH

LH Endometrio�c lesions

Spinal cord

Endometrio�c lesions

Endometrio�c cells

Immune cells

Sensory nerve ending

Primarysensory neuron

Nocicep�ve signal

To spinal cord

Sensory nerve ac�va�on

Neurogenic inflamma�on

Hor

mon

al a

spec

ts o

f end

omet

rios

is

Infla

mm

ator

y an

d no

cice

p�ve

asp

ects

of e

ndom

etri

osis

+

+

(A) (B)(C)

Pro-nocicep�ve mediators

Pro-inflammatorymediators

Degenera�ng�ssue

Peritoneum

Extra-uterine menstrua�on

Inflamma�on

Dorsal root ganglion

Secondarysensory neuron

TRENDS in Pharmacological Sciences

Figure 1. Schematic representation of the complex pathophysiology of endometriosis. Endometriosis is a complex, multifactorial disease. In this figure, hormonal aspects

(A) are opposed to inflammatory and nociceptive components (B,C). Endometriosis is a hormone-dependent disease characterized by the presence and growth of

endometrial tissue in various locations of the pelvic cavity, such as the peritoneal wall and ovaries. Its symptoms are modulated by sex hormones (e.g., estrogen and

progesterone). Sex hormones are under the control of the hypothalamic–pituitary–gonadal axis and gonadotropin-releasing hormone (GnRH) agonists have positive effects

on endometriosis-associated symptoms (A). Beyond hormonal aspects, bleeding endometriotic lesions and retrograde menstruation into the pelvic cavity (extra-uterine

menstruation) induce inflammation (B). Together, endometriotic cells, degenerating tissue, and immune cells release pro-nociceptive mediators, which activate sensory

nerve endings in endometriotic lesions. In response, sensory nerve endings contribute to neurogenic inflammation through the release of proinflammatory mediators (C).

Activation of primary sensory neuron endings leads to the generation of the nociceptive signal, which is conveyed to the central nervous system (CNS). The nociceptive

signal is then integrated in the spinal cord and carried by secondary sensory neurons to higher centers in the brain (B,C). Abbreviations: FSH, follicle-stimulating hormone;

LH, luteinizing hormone.

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271

Macrophages

Mast cells

Spin

al c

ord

Sex

horm

ones

Prolifera�ng endometrio�c cells

Cell deathMenstrua�ng �ssue

Endometrio�c lesions Innate immune system Peripheral nervous system

Sensory nerve endings

Primary sensoryneurons

Dorsal root ganglion

Nocicep�ve signal

Direct sensory nerve s�mula�on

Nervous systemresponse

DAMPs, Iron (OS)

PGE2, Acidosis (H+), Iron (OS)

SP, CGRP

Sensory nerve s�mula�on

NGF, IL-1β, TNF-α, PGE2

(A) (B) (C)

NF-κB

NF-κB

Innate immune system ac�va�on

TRENDS in Pharmacological Sciences

Figure 2. Main pathways involved in the pathogenesis of inflammatory pain in endometriosis. The three main players involved in the pathogenesis of endometriotic pain

are endometriotic lesions (A), the innate immune system (B), and the peripheral nervous system (C). Endometriotic lesions are under the control of sex hormones and

menstruation, proliferation of endometriotic cells, and cell death occur. Mediators released during this process, including prostaglandin E2 (PGE2), H+, and iron, directly

stimulate sensory nerve endings to generate the nociceptive signal. In addition, products released from tissue degeneration [e.g., damage-associated molecular patterns

(DAMPs) or iron] activate the innate immune system through the nuclear factor (NF)-kB pathway. Proinflammatory and pro-nociceptive mediators, such as nerve growth

factor (NGF), interleukin (IL)-1b, tumor necrosis factor alpha (TNF-a) and PGE2, released from activated mast cells and macrophages, are also able to stimulate sensory

nerve endings. In response to stimulation, sensory nerves further increase and maintain inflammation by secreting proinflammatory products, such as substance P (SP) and

calcitonin gene-related peptide (CGRP). Different mediators and receptors are illustrated: IL-1R (IL-1b), TNF-R (TNF-a), TrkA (NGF) and TRPV1 (H+). Abbreviation: OS,

oxidative stress.

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[23,24]. However, the consequences of extra-uterine men-struation in the generation of endometriotic pain have notbeen adequately addressed so far. Here, we propose thattissue degeneration in extra-uterine menstruation has acentral role in the inflammatory response and the genera-tion of pain in endometriosis. In particular, we draw anovel pathophysiological picture of inflammatory pain inendometriosis and summarize key immunological andneurobiological findings in one unifying concept(Figure 2). Although we focus our concept on endometri-osis, similar pathophysiological principles may also beactive in other painful, inflammatory disorders in whichtissue degeneration represents a disease hallmark.

Menstruation causes innate immune activationSimilar to endometrium, endometriotic lesions are governedby endocrine cycles, and cyclic bleeding occurs [15,16]. Men-struation has many features of an inflammatory processand, during the late secretory phase, an increased number ofmacrophages and activated mast cells is observed, whichpersists during menstruation [25]. Moreover, the concentra-tion of innate immune cells is increased in the peritonealfluid of women with endometriosis compared with controls;furthermore, an increased number of activated mast cellsand macrophages is observed in endometriotic lesions[18,26–29]. In our opinion, the activation of the innateimmune system is the first important step in the pathophys-iology of endometriosis. Here, we describe how retrogrademenstruation and the bleeding of endometriotic lesions(extra-uterine menstruation) directly influence peritonealinflammation and activation of the innate immune system.

The highly regulated tissue breakdown occurring duringmenstruation leads to necrosis and apoptosis of endometrial

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cells. Numerous cellular products released during cell deathare well known to induce sterile inflammation by the acti-vation of innate immune cells [30–32]. Thus, menstrualdebris, arising from extra-uterine menstruation, releasesproducts occurring from cell death into the peritoneal cavityand subsequently induces sterile inflammation [33,34]. Thefamily of damage-associated molecular patterns (DAMPs) isthe most well known among these products. DAMP mole-cules include the nuclear protein high-mobility-group box 1(HMGB1), heat shock protein 60 and 70 (HSP60 andHSP70), S100, nucleic acids, and soluble extracellular ma-trix components [35]. Several studies describe the presenceof DAMPs and suggest their implication in endometriosis.For example, HSP70 levels are higher in the serum ofwomen with endometriosis compared with healthywomen, and HSP70 is detected in endometriotic lesions[36,37]. Members of the S100 family of calcium-modulatedproteins (also known as calgranulins) are overexpressed inendometriotic lesions (S100A13) and the peritoneal fluid ofwomen with endometriosis (S100A8), indicating a probablerole in the pathogenesis of endometriosis [38,39]. HMGB1 isexpressed in endometriotic lesions and its role in the acti-vation of innate immunity in endometriosis has alreadybeen suggested [51]. Extracellular DAMPs, such as fibro-nectin, fibrinogen, or hyaluronan, are also described to bepresent in endometriotic lesions and in the peritoneal fluidof women with endometriosis [33,41]. In addition, nucleicacids are released during tissue degeneration and cell death,and high amounts are found in menstrual blood [42].

In addition to DAMPs, iron and oxidative stress are alsomajor products of tissue degeneration and cellular injury.Higher concentrations of iron have been detected in peri-toneal fluid of women with endometriosis compared with

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controls, and iron is described as a main constituent ofendometriosis-associated inflammation [34,43–45]. Inter-estingly, Escherichia coli is found in menstrual blood andperitoneal fluid of women with endometriosis, indicatingnot only a role of DAMPs, but also of pathogen-associatedmolecular patterns (PAMPs) in endometriosis-associatedinflammation. This is supported by higher concentrationsof bacterial endotoxin in menstrual blood and peritonealfluid of women with endometriosis compared with thosewithout endometriosis [46,47].

Products arising from menstrual debris induce the acti-vation of macrophages and mast cells through the predom-inant activation of the proinflammatory transcriptionalfactor nuclear factor (NF)-kB. Indeed, reactive oxygenspecies (ROS) produced subsequently to the iron overloadactivate mast cells and macrophages through NF-kB[40,48,49]. DAMPs and PAMPs are themselves able toactivate NF-kB through their actions on pattern recogni-tion receptors (PRR), a family that includes, among others,toll-like receptors (TLRs) [50]. As recently reviewed, theinteraction between DAMPs and TLRs has a major role inthe pathophysiology of endometriosis [33,51,52]. In endo-metrium, nine TLRs are expressed by immune, epithelial,and stromal cells [53,54]. TLR2 and TLR9 are overex-pressed in the peritoneal fluid of women with endometri-osis, and there is an increased TLR4 expression inendometriotic lesions of women during the secretory phase[55,56]. TLR3 and TLR4 are expressed by dendritic cells,monocytes, and macrophages in endometriotic lesions andare increased in endometriotic lesions compared with en-dometrium [57]. As a consequence of innate immune acti-vation, a range of proinflammatory and pro-nociceptivemediators is released (see below for more detail).

In summary, there is likely to be a central role formenstrual debris, more specifically DAMPs, as triggersof innate immunity in endometriosis. The activation ofmacrophages and mast cells by extra-uterine menstrua-tion leads to the secretion of pro-nociceptive and proin-flammatory mediators, which contribute to the generationof nociceptive signals and the feed-forward loop, maintain-ing and strengthening the peritoneal inflammation(Figure 2).

Peritoneal menstruation triggers sensory nervestimulationSeveral products are released into the peritoneal cavity bydegenerating tissue during extra-uterine menstruation thatboth activate the innate immune system and sensorynerves. Here, we describe how these products can activatesensory nerves, directly through tissue degeneration-relat-ed products or indirectly through innate immune cell acti-vation. The activation of sensory nerve fibers that generateand convey nociceptive signals to the central nervous system(CNS) is an essential step in the pathophysiology of pain.Sensory nerve endings are present in endometriotic lesions,and a positive correlation between the amount of nervefibers and pain symptoms has been described in differenttypes of endometriosis [23,24], which strongly suggests theirsignificance in endometriosis-associated pain.

It is well known that tissue degeneration and cell deathlead to acidification of the tissue [58]. This pH decrease is

sensed by several receptors expressed by the sensory nervefibers. Among them, the transient receptor potential vanil-loid 1 (TRPV1) channel has been widely studied and its rolein inflammatory pain is well known [59]. Increased num-bers of TRPV1-positive nerve fibers are found in endome-triotic lesions of women with CPP compared withendometriotic lesions of women without CPP, supportingthe implication of TRPV1 in endometriosis-associated pain[60]. As described above, extra-uterine menstruation leads,partially through iron overload, to an increased productionof ROS. ROS have a direct pro-nociceptive effect by sensi-tizing sensory nerve fibers, mainly by enhancing TRPV1activity [61–63].

Prostaglandins are produced and released upon cellactivation and tissue damage [64]. They directly activatesensory nerve fibers. Prostaglandin E2 (PGE2) is abun-dantly detected in the peritoneal fluid of women withendometriosis and its role in the pathophysiology of thedisease is well described [65]. Indeed, PGE2 is the mostwell-known lipid mediator that contributes to inflammato-ry pain. PGE2 has a pro-nociceptive effect through thesensitization of sensory nerve fibers [66]. Nonsteroidalanti-inflammatory drugs (NSAIDs) suppress pain symp-toms in inflammatory pain diseases, but they have numer-ous adverse effects upon chronic treatment [67].

In addition to the above-described direct effects, extra-uterine menstruation can also indirectly stimulate sensorynerves by innate immune activation. More inflammatorycells are observed near nerve fibers in women with endo-metriosis and with more severe pain symptoms, suggestinga direct activation of the sensory nerve afferents by im-mune cells and the inflammatory environment [68–70]. Ac-tivated mast cells and macrophages synthesize and secretenumerous proinflammatory and pro-nociceptive media-tors. The concentrations of mediators, such as cytokines,chemokines, complement components, prostanoids, reac-tive oxygen intermediates, hydrolytic enzymes, and growthfactors, are increased in the peritoneal fluid or in endome-triotic lesions [18,51,71]. Among them, interleukin (IL)-1b,tumor necrosis factor alpha (TNF-a), and nerve growthfactor (NGF) are able to directly activate sensory nerveending. Their implication in the generation of pain is welldescribed in several inflammatory diseases and has beenreviewed recently in endometriosis [23,24].

In contrast to other inflammatory pain disorders, theunderlying mechanism of endometriosis-associated painis still incompletely understood. Here, we have summa-rized evidence that extra-uterine menstruation activatesand sensitizes sensory fibers present in lesions through adirect or an indirect action (Figure 2). Together, theconsequences of extra-uterine menstruation are likelythe first triggers for sensory nerve stimulation and,subsequently, the generation of inflammatory pain inendometriosis.

Nervous system response further increases peritonealinflammationNerve fibers themselves also have an active role in themechanism of inflammatory pain by secreting proinflam-matory neuromediators. This mechanism, called neuro-genic inflammation, is observed in several inflammatory

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painful conditions, such as complex regional pain syn-drome, migraine, and arthritis [72–74].

Neurogenic inflammation is also likely to be of relevancein endometriosis [24]. The neuropeptide substance P (SP)is present in the sensory nerve fibers of peritoneal and deepinfiltrating endometriotic lesions [75,76], and calcitoningene-related peptide (CGRP) is detected in peritonealendometriotic lesions [76]. Furthermore, the SP receptorneurokinin 1 receptor (NK1R) is expressed in endometrio-tic lesions [77]. In addition, patients with the NK1R genepolymorphism rs881 and high preoperative pain levelshave a higher disease free-survival probability comparedwith patients with the wild type genotype [78]. Despite thefact that the consequence of rs881 polymorphism on NK1Rfunction is not known, a loss-of-function could be speculat-ed. This would further support a pathogenic role of SP/NK1R and the existence of neurogenic inflammation inendometriosis [78]. Neurogenic inflammation may cause avicious circle in endometriosis: SP stimulates mast cellsand macrophages and induces the release of proinflamma-tory cytokines, such as TNF-a, IL-6, and IL-8 [79]. CGRPinduces mast cell degranulation and edema by vasodila-tory effects [72]. Interestingly, TNF-a concentration isincreased in the peritoneal fluid of women with endome-triosis-associated nerves in lesions compared with womenwith endometriosis but without associated nerve fibers[70]. Therefore, the increased number of nerve fibersmay have proinflammatory effects, suggesting the exis-tence of a neurogenic inflammatory process.

In summary, there is accumulating evidence for neuro-genic inflammation in endometriosis. Proinflammatorypeptides released by sensory nerve fibers may completea vicious cycle between sensory nerve activation by extra-uterine menstruation and the nervous system response,which further increases and maintains inflammation(Figure 2).

Concluding remarksAs outlined above, endometriosis is not only a hormone-dependent disease, but also an inflammatory disease.However, the current standard of care poorly reflects thiscomplexity because it mainly comprises combined oralcontraceptives, progestins, or short-term treatment withgonadotropin-releasing hormone (GnRH) agonists. Thesetreatment modalities only indirectly interfere with inflam-matory pain by suppressing the lesions and/or modulatingthe menstrual cycle and, consequently, the amount ofmenstruating tissue, although anti-nociceptive and im-mune-modulatory effects of gonadal hormones have beendescribed (reviewed in [80,81]). Owing to the molecularnature of endometriosis, the limited efficacy of currentlyavailable hormonal drugs, and their potential adverseeffects, there is a high medical need for innovative andbetter treatment.

What would such innovative treatment options looklike? What are the general treatment requirements andwhich disease mechanisms could potentially be targeted?Women with endometriosis are in general young, other-wise healthy women of reproductive age. Although somewomen with endometriosis are asymptomatic, the preva-lence of endometriosis in women with CPP and/or

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infertility is high and varies from 30 to 50% [82,83]. Thesesymptomatic women are in need of safe and convenientmedical and/or surgical treatment, which is effective in thelong term. This includes ideally: (i) a lack of sex steroiddeprivation-linked adverse effects (e.g., hot flushes, reduc-tion in bone mineral density, etc.); (ii) unchanged men-strual cycle; (iii) no negative impact on fertility; and (iv) notoxicity to reproduction [84,85]. Given that the main trig-ger for endometriotic pain is extra-uterine menstruationand the subsequent peritoneal inflammation (i.e., a pe-ripheral stimulus), we think that interfering with theperipheral nervous system is advisable. This would reducethe risk of unwanted CNS effects, including potentialnegative effects on neuroplasticity. In addition, actingon peripheral targets has the potential to reverse centralchanges (e.g., central sensitization) occurring in womenwith endometriosis [86]. Peripheral input activity has animportant role in the maintenance of central changes inchronic painful diseases and treatment modalities target-ing peripheral causes of ongoing chronic pain might haveshort-term benefits in patients (reviewed in [87,88]).

Besides safety, innovative drugs have to demonstrateclinically meaningful efficacy. In controlled clinical trials,the currently available treatment options, such as com-bined oral contraceptives, progestins, or GnRH agonists,show a marked reduction in pain experience. However,high mean surgery numbers per lifetime (2.8) and ratherlow rates of patient satisfaction with the available com-pounds are indicators that efficacy in daily clinical prac-tice may differ from that observed in controlled clinicaltrials, indicating the need for improved efficacy [89]. Inaddition, there is also a need for a faster response. Peakefficacy following hormonal treatment of endometriosis iscurrently only achieved with a delay of several weeks[90], most likely due to the indirect mechanism of actionof sex hormone-based drugs. Compounds with the poten-tial to rapidly improve patients’ symptoms within a fewdays will likely further improve their quality of lifesignificantly.

Based on our opinion that extra-uterine menstruationhas a central role in inflammation and pain in endome-triosis, we believe that compounds that act on more thanone of the above defined three pathways will have themost significant therapeutic potential. In this way, apromising target could be involved in both the stimula-tion of sensory nerve endings and the activation and/orstimulation of innate immune cells. For example, thiscould be achieved by dampening oxidative stress or block-ing the action of proinflammatory and pro-nociceptivecytokines (e.g., IL-1b or TNFa). The key next steps forthe scientific community is to identify effective and safecompounds against the proposed pathways and to testthem in clinical trials to bring preclinically validatedtargets to the next level.

AcknowledgmentsWe thank Jens Nagel and Damian O’Connell for critical comments andreviewing the manuscript.

Disclaimer statementA.L-B and T.M.Z. are employees of Bayer AG; T.M.Z. is also a shareholder.

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