Friday, January 22, 2010

Random Thought: Mass balances and drug dosing

It happened again. I have mentioned many times in the mass and energy balance class that I could not understand how drug dosing is done.   I suggested that this needed attention by the medical profession (as brazen as this seems!)    That is,  the adult dose for almost all drugs, is the same -- be it one, two, three or 4 times per day.  I could not help but wonder how small women and really large men would need the same dose when a simple mass balance tells us that if it some systemic concentration of drug is needed for efficacy, then the dose should scale roughly as weight.  If there is partitioning of the compound in different types of tissue (e.g., fat tissue which would be hydrophobic), then perhaps a more nuanced criterion is necessary.  However, in either case, all adults are not equal (despite the claim in the founding documents of the US!)

Well, as reported by the New York Times (, a new paper in Lancet ( ), has suggested the need to tailor doses of antibiotics to a person’s size.  They note that obese patients may not get enough drug to clear an infection and that underdosing could be contributing to antibiotic resistance of various bacteria.  There is an accompanying editorial (, which I can also not see (##!!) that suggests the need to study this issue with careful trials.

Beyond the irresistible “duh”, this is actually a very interesting issue for chemical engineers.  The first is that there are good reasons to have FDA approved dosing rules.  You would not want physicians to be “winging it” for every patient -- particularly if they could not solve the transient mass balance equation with simple elimination and reaction terms.  Further, it is critical that patients take the correct dosing.  So three small pills and one big pill, taken every 3 hours is probably problematic for most people (except engineers -- we can count and if you are a student you don’t sleep!).  However, the question of if a particular condition willrespond better to a constant concentration or peaks and valleys would seem not to have a general answer.  Maybe two big doses, is better than four doses of 1/2 the size.  

Looking a bit further afield, we could consider how medicine is likely to progress and how chemical engineers will be involved.  Various “artificial pancreas” devices are in different stages of development.  Chemical engineering plays an essential role in glucose sensing, determination of the “control scheme” and materials necessary for construction.  The implantable drug delivery system, either permanent or temporary, idea could be extended to many more diseases with the essential advantage being feedback control.  Either the drug level could be controlled or the dose could be altered in response to levels of something that is a response to the disease such as a specific cytokine or a blood toxin.  

Cochlear implants have become a common and successful treatment to restore hearing.  Soon there will be vision devices.  This technology, which allows linking of electronic and electromechanical devices, to nerves and other human tissue, could allow blood pressure monitoring, heart rate analysis, or other medical measures that determine health of the patient on a continuous basis.  We might also envision “health stations” for minor ailments where various sensors are used to check out patients and provide a first level screening that helps decide if a physician is needed.  The physician counseling could be remove -- although this invokes a vision of a “call center” for computer software …. -- and the consultations could be with the next available person from a large pool rather than the one overworked Doc at MedPoint.  This approach may be distasteful to some, but potentially could save a lot of money with out limiting eventual use of the best treatments.   

All of these technologies will require the skills of chemical engineers and there are certain questions that ONLY chemical engineers can answer.  Thus chemical engineers will play a key role in the future of health care both to enable treatments that are not yet possible and to save money to allow for care of many more people. 

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