Dr. John Jurist, Tuesday, 3-27-12 March 27, 2012Posted by The Space Show in Uncategorized.
Tags: astronaut medical information., bed rest studies, bisphosphonates, bone loss, bone mass, bone structure, centrifuge, Dr. John Jurist, extra-terrestrials, gene modification, human spaceflight, lunar gravity, Mars gravity., microgravity, microgravity exercise, muscular-skeletal system, NASA, orthopedic surgeons, penguin suit, piezoelectric, Pyrophosphates, radiation, science fiction, variable gravity research station in LEO
Dr. John Jurist, Tuesday, 3-27-12
Part 1, Bisphosphonate Usage in Microgravity
Guest: Dr. John Jurist. Topics: This is the first in a series of programs focused on the use of bisphosphonates to mitigate bone loss in long duration spaceflight. You are invited to comment, ask questions, & discuss the Space Show program/guest(s) on the Space Show blog, http://thespaceshow.wordpress.com. Comments, questions, & any discussion must be relevant & applicable to Space Show programming. Transcripts of Space Show programs are not permitted without prior written consent from The Space Show (even if for personal use) & are a violation of the Space Show copyright. We welcomed back Dr. John Jurist for the first of several programs devoted to understanding bone loss in microgravity & the use of prescription the bisphosphonate family of drugs to mitigate the problem. Dr. Jurist provided talking points which you can find below this summary on The Space Show blog. A significant portion of the second segment is devoted to understanding these talking points so follow along. We started our discussion with an explanation of the bone loss problem facing those in microgravity. Following this brief overview, Dr. Jurist told us about bisphosphonates, the origins of this drug, how it was developed, how its usage, what some of the side effects are, some of the trials that have been associated with the drug, & space/NASA studies. Later in this nearly hour long segment, we talked about the human population & what happens to our bone mass at various stages in our life. We talked about gender, racial, & ethnic differences in human bone mass issues. As you will hear, there are gender & other differences among our diverse population. This led us to asking about using targeted genetics in selecting crews for longer spaceflight missions. Genetic modification was mentioned, especially for the long term, but for the near term, looking for better exercise routines & pharmaceutical modification/treatment seem to hold the most promise. Dr. Jurist then talked about bisphosphonate side effects. Fast transit time to Mars or a long duration mission destination came up. As you will hear, even if you get to Mars faster, you still have the problem on the surface of Mars. The big problem is we have no credible information on what the gravity prescription for humans needs to be so we do not know if Martian gravity is sufficient to mitigate the problem. Next, we focused on the need for an orbiting centrifuge, we talked about the size & spin rate of the centrifuge, & what to actually test for in experiments. Dr. Jurist suggested that starting with 1/6th lunar gravity made sense because if it was sufficient, we would know that anything stronger than 1/6th would work. If not, we should probably next experiment with Martian gravity which is about 1/3rd Earth gravity.
In our second segment, Dr. Jurist referred to his talking points which you will find on The Space Show blog. We went line by line so follow along with us. Despite interruptions, John stayed on topic with the talking points. Listener questions asked about how gravity & having a load on bones works & what happens in space when that load is no longer present. We talked about the time prescription as well as the load factor (G force). Toward the end, John told us about a reviewed radiation article from 2010 showing that radiation impacted bone loss so the issue in space is more complex than just microgravity. Near the end, John got a question asking about ETs & their thin, pencil like shape making them perfectly designed for microgravity. This was both an interesting & fun discussion, bordering on science fiction. Another question asked about fish, reptilian, sea mammal, & bird bones. Our final topic dealt with the release of personal astronaut medical information & others & the ways for private information to be legally released to the public.
Please post your comments/questions on The Space Show blog URL above. Email for Dr. Jurist can be sent to me for forwarding.
Dr. John Jurist Bisphosphonate Talking Points:
Bisphosphonate show Tues 8:00-9:30 MDT Talking Points
Introduction: Bone is incredibly complex with many inter-related factorsGoal: Brief outline of cellular, structural, and functional aspects of bone metabolism in order to help listeners better understand and appreciate the potential role of bisphosphonates (which is the subject of a future show)
Bone Function: Two types of bone – compact and trabecular: Compact is outer shell, very dense, and fairly strong
Trabecular – also called spongy, contains spicules, plates, and struts of bone tissue with lots of vascular and fatty marrow, called trabecular
Remodeling: Bone is constantly being turned over or remodeled with portions being torn down and constituents dumped into bloodstream and other portions being built up from nutrients supplied by blood
Very rapid bone loss can lead to kidney stone formation from calcium dumped into blood
Turnover rate and the volume of total skeleton undergoing remodeling at any given time tends to go down with increasing age (remember this – important)
Balance between formation and resorption (tearing down) also tends to be lost after about age 30, lack of activity, certain hormonal imbalances, diseases, etc. with resorption predominating – one rationale for estrogen replacement therapy after menopause
Coordination between formation and resorption partly controlled by mechanical strains on bone tissue (called Wolff’s Law and issue in microgravity) – some evidence that mechanism is electrical in nature – rationale for electric and electromagnetic bone growth stimulators used to treat problematic fractures and improve incorporation of bone grafts
Cells: Stem cells differentiate (or specialize) into osteoprogenitor cells which differentiate into osteoblasts which are the bone formers
Osteoblasts create and secrete a protein and polysaccharide matrix called osteoid which is then calcified later to form the stuff we call bone
Interference in that calcification leads to rickets in children and osteomalacia (Greek for soft bones) in adults
Osteomalacia is often a result of interference in Vitamin D metabolism or dietary deficiencies
During bone formation, some osteoblasts “paint” themselves into a corner and get surrounded by bone – they are then called osteocytes and somehow maintain bone tissue
If osteocytes die in a volume of bone, that devitalized bone no longer undergoes remodeling but can eventually be replaced by a process called creeping substitution (remember comment about remodeling volume going down with age)
Devitalized bone can accumulate fatigue defects and eventually suffer fatigue fractures (called stress fractures – common in military recruits and deconditioned people who recondition too rapidly) if the defects are not replaced by new bone during creeping substitution
Other cells, with multiple nuclei and called osteoclasts, differentiate from marrow cells (monocytes and macrophages) which in turn differentiate from stem cells
Osteoclasts are the bone resorbers and destroyers
Autoradiographic studies show that their nuclei also came from osteocytes as well – presumably as they are resorbed during bone destruction process
Osteoclasts are related to multinucleated “giant cells” which are associated with inflammation in ways I do not understand
Bisphosphonates: Used over past 20-30 years in clinical medicine – brands include Actonel, Boniva, Fosamax, Aredia, etc.
Multiple mechanisms with different dose-response properties results in clinicians juggling doses and brands to get effects they want without effects they do not want
Bind to hydroxyapatite – the mineralized portion of bone tissue (calcified osteoid)
Inhibit bone resorption by inhibiting osteoclastic activity
Inhibition of resorption signals osteoblasts to slow down formation rates after a delay (but increases their lifetimes (anti-apotosis)
Bisphosphonates also inhibit mineralization of osteoid – especially at higher doses
That eventually reduces turnover rate and can lead to fatigue fractures with prolonged treatment (remember to call 1-800-BAD-DRUG)
Orthopedic surgeon (Frost atHenryFordHospital) figured out that cyclic therapy exploits delay in osteoblast inhibition relative to osteoclast activity and results in better bone mass gains
Bisphosphonates used in oncology to preserve bone and reduce pathological fractures with osteolytic tumors such as breast and prostate cancers. Also to counter hormonal blocking therapy used for prostate and breast cancers, but side effects increase with duration of treatment
Interesting sidelight with oncology is some evidence that bisphosphonates reduce risk of tumor metastasis to bone and other sites (somehow involving same signaling pathway as giant cells?)
Oral bisphosphonates cause stomach upset – advise staying upright for an hour or two after taking pill – how does this affect microgravity exposure?
Intravenous bisphosphonates get around oral gastric upset, but placing line or port has risks too (most oncology patients have ports during active treatment)
Also cause osteonecrosis – especially jaw which I believe is septic or infective – and atrial fibrillation (talk to Bill Rowe on this one since he is interested in heart physiology in space)
Bone quality: Turnover rates and percent of skeleton not devitalized important factors in determining bone strength – measuring bone mass by DEXA or other means is not the be all and end all in evaluating bone strength and risk of fracture.
My Goal: General background on bone physiology and function to aid in appreciating upcoming show(s) on bisphosphonates in space medicine