Is progesterone a worthy candidate as a novel therapy for traumatic brain injury?

Abstract

Although progesterone is critical to a healthy pregnancy, it is now known to have other important functions as well. Recent research demonstrates that this hormone is also a potent neurosteroid that can protect damaged cells in the central and peripheral nervous systems and has rapid actions that go well beyond its effects on the classical intranuclear progesterone receptor. Based on years of preclinical research demonstrating its safety and effectiveness in animal models of central nervous system injury the hormone was recently tested in two Phase II clinical trials for traumatic brain injury (TBI). A US National Institutes of Health-sponsored, nationwide Phase III clinical trial is now evaluating progesterone for moderate-to-severe TBI in 1200 patients. An industry-sponsored Phase III international trial is also under way, and planning for a trial using progesterone to treat pediatric brain injury has begun. Preclinical data suggest that progesterone may also be effective in stroke and some neurodegenerative disorders.

Background

Traumatic brain injury (TBI) is an important, clinical problem in the United States and worldwide, but, after almost, 50 years of research, no safe, clinically effective treatment has been found that can reduce mortality and morbidity and improve functional outcome. TBI has received more attention recently because of its high incidence in combat, casualties in Iraq and Afghanistan. US Department of Defense statistics indicate that as many as 30% of wounded soldiers seen at Walter Reed Army Hospital have suffered a TBI, a finding which has stimulated government interest in developing a treatment for this complex disorder. Both pediatric and geriatric TBIs are also on the rise as children of both sexes become more involved in contact, sports and as the elderly live longer, drive more, and become more susceptible to auto accidents and falls.

A neuroprotective treatment for stroke has also proven elusive. Well over 100 clinical trials for this incapacitating disease have yielded little substantive benefit for patients. Aside from tissue plasminogen activator (tPA), which can be given to only about 3% to 5% of stroke victims, and only in the first, 3 to 4 hours after stroke onset, nothing that shows any evidence of reduction of infarct size and neuroprotection is available for clinical use.

Progesterone as a treatment for brain injury?

Progesterone (PROG) is the bright exception to this grim picture. A search of PubMed using the keywords “progesterone, brain injury” yields over 200 publications from more than 25 independent, laboratories around the world using 22 different injury models demonstrating that PROG treatment can have beneficial effects leading to substantial and sustained improvements in cytological, morphological and functional outcomes.

We now know that, PROG given to both males and females after central nervous system (CNS) injury can: cross the blood-brain barrier (BBB)^1,2 and reduce edema levels^3-5; reduce lipid peroxidation and isoprostanes, which contribute to post-injury ischemic conditions⁶; generate metabolites which reduce proapoptotic and increase antiapoptotic enzymes⁷ and decrease the expression of proinflammatory genes and their protein products^8,9; modify the expression of vascular endothelial growth factor (VRGF) and brain-derived neurotrophic growth factor (BDNF)^10-13; influence the expression of aquaporins implicated in the resolution of edema; protect neurons distal to the injury which would normally die¹⁴; and enhance oligodendrocyte-induced remyelination in young and aged rats with demyelinating disorders.^15,16 PROG has a substantial treatment window and can be given up to 24 hours after injury and still show beneficial effects in animal and clinical TBI.^5,17 Importantly, it has been shown to produce significant sparing of cognitive, sensory, and spatial learning performance in mature and aged rats following bilateral brain injuries.¹⁸ A number of reviews discuss these data and highlight the fact that PROG and its key metabolites play a critical role in both normal development and repair of the CNS after injury (see Figure 1 for a diagram of the metabolic pathways of this hormone).^19-25

Figure 1. Steps in the biosynthesis of progesterone and some of its metabolites.

Thus far much of our group's research on PROG and its metabolites has focused on the treatment of TBI. ^8,14,26-29 This line of research originated when we found that following bilateral contusion injury to the medial frontal cortex (MFC) in young adult, male and female rats, 3 to 5 days of post-injury treatment, with PROG significantly reduced cerebral edema and improved spatial learning and sensory performance compared with controls given vehicle alone.^30,31

Progesterone tested successfully in two phase II clinical trials

The first successful clinical trial for the treatment of TBI in more than 30 years of research was published in 2007.¹⁷ This NINDS-sponsored, Phase Ha single-center clinical trial for PROG in the treatment of moderate-to-severe adult TBI found that the mortality rate among patients given PROG intravenously for 3 days post-injury was less than half that, of controls (13.6% versus 30.4%). Thirty-day functional outcomes for moderately injured patients in the PROG group were significantly better than those for the placebo group. An NIH-appointed Data Safety Monitoring Board found no serious adverse events attributable to PROG treatment. A second independent, randomized doubleblind study from China tested PROG in 159 patients with severe TBI given a course of intramuscular injections for 5 days. The investigators reported similar beneficial outcomes on morbidity and mortality at both 30 days and 6 months after injury, again without any serious adverse events caused by the treatment.³² A National Institute of Neurological Disorders and Stroke is supporting a Phase III, 31 -center, doubleblind, randomized clinical trial for TBI which is now enrolling patients across the United States (ProTECT III: NCT00822900), and a second trial (SyNAPSe: NCT01143064) is testing PROG in over 140 centers worldwide. The 22-center Pediatric Emergency Care Applied Research Network (PECARN) is in the early stages of planning for a Phase I/II clinical trial using PROG in brain-injured children.

The continuing stream of positive results seem almost too good to be true―especially in light of the history of failures to find an effective neuroprotective agent. Some investigators^25,33 have expressed concern that many, if not most, preclinical animal studies in TBI lack direct, translation to clinical relevance because they fail to meet certain standards similar to the Stroke Therapy Academic Industry Roundtable (STAIR) recommendations.³⁴ While no one study may be able to meet all the STAIR recommendations, it is important to note that in the aggregate, the large number of studies on PROG do, in fact, meet such criteria as:

• Dose-response studies

• Statistical power analyses to determine sample size(s)

• Comparison with other agents thought to be effective, their antagonists, or knockout technologies to elucidate mechanisms

• Histological and functional outcome measurements to examine short- and long-term effects

• Monitoring of relevant variables during surgery

• Studies in both males and females

• Studies in different, models and species

• Replication of effects across laboratories

(These criteria are derived from recommendations proposed by Loane and .Fadcn.³³ They arc similar to the STAIR recommendations for use in testing new drugs for the treatment, of stroke).

Much of the growing support for PROG as a potential treatment is likely based on its high safety profile and evidence of efficacy in animal and human testing, but, it, will be at least several more years before any conclusions concerning its neuroprotective benefits in largescale testing can be fully confirmed.

Progesterone in stroke and neurodegenerative disorders

Stroke has overlapping pathophysiological mechanisms with TBI, and the preclinical stroke data and recent, human studies in 1131 support a potential role for PROG in acute stroke. Recently we reported significant neuroprotective effects of acute post-injury administration of PROG in an adult rat model of permanent and transient (2 h) middle cerebral artery occlusion stroke.³⁵-³⁶ In different models of cerebral ischemia, PROG can significantly reduce the area of necrotic cell death and improve behavioral outcomes.³⁷ Our findings corroborated other studies showing PROG to be neuroprotective following global ischemia in cats,^38,39 and transient focal ischemia in rats.^37,40 Several reviews and original research papers^13,22,41-45 on the use of neurosteroids in stroke note favorable outcomes in reduction of infarct size leading to better functional status.⁴²

Nevertheless, TBI and stroke are very different diseases, and there is no guarantee that PROG treatment will work in human stroke. Before going forward to clinical application, it, will be important to determine whether PROG treatment combined with tPA is safe and whether it has positive or negative outcomes. Our own recent experiments suggest that PROG may upregulate the expression of endogenous tPA and extend its window of effectiveness up to at least, 6 hours, but this will need to be confirmed by further research.⁹ As of this writing, no clinical trials using PROG or its metabolites are being planned for stroke patients.

There is now evidence that PROG and some of its metabolites may be beneficial for other neurodegenerative disorders. Over the last, 10 years, several groups have reported that PROG exerts beneficial effects in spinal cord injury, including enhanced remyelination and improved motor function.^21,46-50 In a streptozotocininduced diabetic neuropathy model in rat, Leonelli et al⁵¹ showed that treatment with PROG and some of its metabolites sustains nerve conduction velocity, restores skin innervation, and maintains sensitivity to thermal stimulation.

Treatment with various allopregnanolonc (ALLO; a metabolite of PROG) regimens in an animal model of Niemann-Pick type C disease, an irreversibly fatal developmental neurodegenerative illness, may delay the onset of symptoms (tremor, ataxia, weight loss).^52-54 Another neurodegenerative condition, experimental autoimmune encephalomyelitis (RAF,), a model of multiple sclerosis, may be amenable to treatment with PROG or its metabolites.⁵⁵

From a psychiatric perspective there are some additional advantages to considering PROG and its metabolites as therapeutic agents following brain injury caused by trauma or stroke. Both PROG and ALLO are present, in the serum and brain. ALLO is metabolized when PROG is converted to dihydroprogesterone by 5α-reductase and then into ALLO by 3α;-hydroxysteroid dehydrogenase. ALLO is being evaluated because of its effects on the expression and release Y-amin°butyric acid (GABA)_a, an inhibitory neurotransmitter which can block or reduce post-injury seizure activity and its resultant excitotoxicity.⁵⁶ ALLO but, not PROG is thought to act directly as a ligand for the GABA_a receptor and some of its isoforms.^56,57

A major component of TBI and stroke is depression and anxiety,^58-61 and there is growing evidence that administration of PROG and ALLO may play a role in reducing TBI-related symptoms in animal models of these disorders. The anxiolytic and antidepressant effects of PROG and ALLO can even be observed in progesterone receptor (PR) knockout mouse models, suggesting that the classical PRs are not necessarily implicated directly in the beneficial outcomes of the treatment.⁶² However, others have claimed that when some PR antagonists have been used, depression-like behaviors will become more obvious,⁶³ so the issue may be more complex than one might, think. Nonetheless, the literature can be taken to suggest that PROG and ALLO, acting as neurosteroids through a variety of different receptor mechanisms, do play a role in improving anxiety and depression-like symptoms that follow CNS trauma.

It is also interesting to note that cerebrospinal fluid (CSF) levels of ALLO are decreased in patients diagnosed with PTSD or unipolar depression. Again, in animal models administration of ALLO (which is also synthesized in the brain) reduces aggression, normalizes fear responses, and decreases anxiety behaviors similar to the effects of giving fluoxetine or similar drugs.^64,65

Finally, endogenous levels of pregnenalone (a precursor of PROG and ALLO) have been associated with increased analgesic and decreased nociceptive effects, likely by modulating the GABA_a receptor. In one clinical study,⁶⁶ veterans of the Iraq war reporting low back pain, chest pain, muscle soreness, and headaches had their serum ALLO and DHRA levels measured by gas chromatography (GC/MS) or radioimmunoassay. The investigators found an inverse relationship between ALLO levels and self-reports of low back pain. Taking all the data together, it seems reasonable to suggest that the clinical use of neurosteroids might, provide a number of psychological benefits that are not currently available with other drugs used in the treatment of TBI or stroke. The data showing that PROG and ALLO are neuroprotective is now fairly extensive, but there is growing evidence that, depending on dose and timing of administration, these same treatments may also be able to reduce some of the long-term consequences of brain damage such as aggression, high anxiety, depression, and cognitive disorders without a lot of concomitant negative side effects, while at other times, fluctuations in levels of ALLO can increase negative mood symptoms.⁶⁷

The final story on the role of PROG and its metabolites in altering mood and cognition in brain-injured patients can be verified only by clinical trial evaluation involving more chronic treatments and longer-term follow-up studies than are typically done when studying the acutestage outcomes of TBI or stroke.

Besides its neuroprotective effects, PROG also shows beneficial effects in other organs that are affected by brain and corporal trauma. For example, PROG administration following trauma-hemorrhage has been reported to ameliorate the proinflammatory response and, subsequently, hepatocellular injury via direct action on immunocompetent cells.⁶⁸ It has also been shown to significantly reduce inflammation in other tissue that is directly or indirectly affected by TBI or stroke. It has recently been demonstrated that post-injury treatment with PROG can reduce the expression of inflammatory cytokines in the gut, spleen, liver, and heart, thus reducing the inflammatory cascade of events leading to additional, secondary neuronal and cellular destruction.^44,69-71

Progesterone is a pleiotropic agent for brain injury

How can PROG have so many different salutary effects on tissue repair? PROG is a naturally occurring hormone found in both males and females, although healthy, premenopausal females will have higher levels than males. However, unlike other steroid hormones, PROG is synthesized directly in the brain by oligodendrocytes and neurons,^47,72 so it can be considered a “neurosteroid” beyond its role as a sex hormone.

In pregnant, females, PROG levels increase 10- to 15-fold and remain high throughout, gestation, but in humans, within 1 to 2 hours after parturition, levels drop precipitously. Why? It is becoming more evident that PROG’s multiple mechanisms of action have evolved primarily to protect, the developing fetus from oxidative stress and immune-inflammatory rejection reactions. The hormone is now thought to play a critical role in neuronal-glial signaling and normal neuronal development. Many of the processes of tissue repair, including cells in the CNS, recapitulate steps that take place during development.^73,74

Based on what we have seen following treatment with PROG in adult subjects with brain injury, we hypothesize that its role in protecting the fetus during development recapitulates its effects in the treatment of traumatic and degenerative disorders of the brain and CNS. It is also important to note that, PROG and its metabolites such as ALLO are pleiotropic―they act at multiple receptor sites, not just, at the classical intranuclear PR. These mechanisms are discussed in a number of recent reviews that highlight the rapid, nongenomic actions of PROG in the CNS and other tissue affected by injur}' or degenerative diseases.^75,76

Conclusion

Many other drugs have failed in clinical trial because they seem to act only at one or a few receptor sites, or only affect, the expression of genes implicated in the injury cascade. To reiterate, it is suggested here that TBI and stroke are systemic diseases that produce wide-spread negative symptoms in many tissue and organ systems throughout the body. An agent or drug that acts systemically to reduce the inflammatory cascade, while at the same time providing trophic support, to damaged nerve cells, is more likely to have a beneficial outcome than an agent, that acts only at a single target in the brain. This is where PROG seems to shine as a therapeutic agent.

Over 25 years of preclinical research by our laboratory and now many others have identified and defined many of the physiological mechanisms underlying PROG’s benefits. Given its relatively high safety profile, ease of administration, low cost, and ready availability, PROG and its metabolites should be considered as a potential treatment, option―especially because in brain injury little else is currently available.