Adam J. Smith‡
*a,
Seol-Hee Kima,
Jun Tanb,
Kevin B. Sneedc,
Paul R. Sanberga,
Cesar V. Borlongana and
R. Douglas Shytlea
aCenter of Excellence for Aging and Brain Repair, Department of Neurosurgery and Brain Repair, Morsani College of Medicine, University of South Florida, Tampa, FL, USA
bRashid Laboratory for Developmental Neurobiology, Silver Child Development Center, Department of Psychiatry and Behavioral Neurosciences, Morsani College of Medicine, University of South Florida, Tampa, FL, USA
cCollege of Pharmacy, University of South Florida, Tampa, FL, USA
First published on 18th February 2014
Despite its narrow therapeutic window, lithium is still regarded as the gold standard comparator and benchmark treatment for mania. Recent attempts to find new drugs with similar therapeutic activities have yielded new chemical entities. However, these potential new drugs have yet to match the many bioactivities attributable to lithium's efficacy for the treatment of neuropsychiatric diseases. Consequently, an intense effort for re-engineering lithium therapeutics using crystal engineering is currently underway. We sought to improve the likelihood of success of these endeavors by evaluating the pharmacokinetics of previously unexplored lithium salts with organic anions (lithium salicylate and lithium lactate). We report that these lithium salts exhibit profoundly different pharmacokinetics compared to the more common FDA approved salt, lithium carbonate, in rats. Remarkably, lithium salicylate produced elevated plasma and brain levels of lithium beyond 48 hours post-dose without the sharp peak that contributes to the toxicity problems of current lithium therapeutics. These findings could be important for the development of the next generation of lithium therapeutics.
Recently, there have been efforts to find a lithium mimetic with improved safety.11,12 It is our opinion that this use of the term “lithium mimetic” is somewhat misleading since none of these new chemical entities have matched lithium's polypharmacological mechanisms of action for the treatment of neuropsychiatric diseases. In particular, lithium therapeutics are deemed gold standard for treatment of mania, thus optimizing their safety and efficacy should have wide-ranging clinical applications.
Alternatively, others have used crystal engineering techniques to re-engineer lithium therapeutics by creating novel ionic cocrystals of lithium salts.7,13,14 We argue that cocrystallization represents a low risk, low cost approach with the most potential for achieving the desired therapeutic outcome for many reasons. For example, the active pharmaceutical ingredient (API) in this crystal engineering approach remains lithium, which is already FDA-approved with a long history of use in medicine. Also, the FDA has just issued a guidance for industry regarding the regulation of pharmaceutical cocrystals that includes an expedited pathway for their approval.15 Thus, the cost to bring a lithium cocrystal to market will likely be significantly lower than that of a new drug.
An important step in the crystal engineering of ionic cocrystals of lithium is the selection of the most appropriate parent lithium salt. One obvious consideration that has already been identified is that the anion of the lithium salt should be pharmaceutically acceptable.7 However, another important factor is pharmacokinetics. Often, lithium salts are assumed to dissociate following oral administration leading to very similar plasma and brain levels of lithium. In fact, one study compared lithium carbonate, lithium chloride, and lithium orotate in rats.16 This author found no differences in the uptake, distribution, and excretion of the lithium ion. Still, due to the complex nature of the pharmacokinetics of multi-component materials, we decided to evaluate the plasma and brain pharmacokinetics of two previously unexplored salts of lithium that seemed to be good candidates for crystal engineering endeavors: lithium salicylate and lithium lactate. Our findings are described in the report herein.
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Fig. 1 Pharmacokinetic curves. Lithium measurements are plotted as mean ± SEM (* P < 0.05, ** P < 0.01, *** P < 0.001). |
Lithium lactate resulted in elevated lithium plasma levels at 2 hours but peaked at 24 hours post-dose and was eliminated rapidly. In contrast, lithium salicylate produced elevated lithium plasma levels through the first 48 hours and was eliminated slowly. Interestingly, both formulations produced elevated brain levels only at 24 and 48 hours post-dose. Table 1 shows some pertinent pharmacokinetic parameters in our experiment. However, these estimates should be used as preliminary indicators since only four carefully selected time points were utilized to limit the use of animals as much as possible. The plasma area under curve (AUC) for lithium lactate was higher than lithium salicylate. However, the brain AUC for lithium salicylate was slightly higher than lithium lactate. Because we utilized the same experimental protocol and time points for our pharmacokinetics study as previously used by Smith et al. for lithium carbonate,7 this allowed the determination of the relative bioavailability (Frel) of lithium salicylate and lithium lactate compared to lithium carbonate (Table 1). The relative bioavailability of both lithium salicylate and lithium lactate were lower than lithium carbonate. The plasma and brain Frel of lithium salicylate was 0.35 and 0.54, respectively. The plasma and brain Frel of lithium lactate was 0.45 and 0.54, respectively.
Lithium salicylate | Lithium lactate | |||
---|---|---|---|---|
Plasma | Brain | Plasma | Brain | |
TMAX (hour) | 24 ± 0.0 | 48 ± 0.0 | 24 ± 0.0 | 24 ± 0.0 |
CMAX (μg mL−1 or μg g−1) | 2.21 ± 0.10 | 2.89 ± 0.13 | 4.54 ± 0.59 | 3.87 ± 0.40 |
AUC(0–72) (μg h mL−1 or g−1) | 121.8 ± 5.71 | 153.1 ± 7.66 | 157.2 ± 19.64 | 152.0 ± 15.90 |
Frel (vs. lithium carbonate) | 0.35 | 0.54 | 0.45 | 0.54 |
Interestingly, we found that lithium salicylate exhibited an unexpected pharmacokinetic profile that is unlike any other lithium salt reported in the literature to date. The known toxicity issues of FDA approved lithium salts could be exacerbated by their pharmacokinetics given its narrow therapeutic window. We previously reported that lithium carbonate peaks rapidly and is eliminated within 48 hours.7 Comparatively, both of the lithium salts in our present study underperformed lithium carbonate from bioavailability standpoints. However, given that oral bioavailability is not a problem with lithium therapeutics17,18 we don't anticipate that this discrepancy will disqualify either of these salts for development as drugs. In fact, we argue that the plateau plasma levels observed in this study of lithium salicylate could improve the safety of lithium therapy and, consequently, improve patient compliance. This is supported by previous investigators who suggested that an ideal lithium preparation would attenuate high blood level peaks and exhibit gradually declining blood concentrations.19 Encouragingly, this is precisely the pharmacokinetic profile that was produced by lithium salicylate in our study (Fig. 1). Previous attempts at formulating proprietary controlled release lithium therapeutics have been somewhat successful at prolonging lithium plasma levels.20 Nonetheless, these formulations still produced the initial plasma spike attributable to toxicity problems observed in lithium therapy. We also found that although lithium salicylate produced comparatively lower plasma lithium exposure than lithium lactate, it produced better brain exposure. Thus, biodistribution also appears to be affected by the choice of anion. Future studies are required to explain the mechanisms behind these observed phenomenons.
Indeed, these pharmacokinetic differences were unexpected since both lithium salts were administered fully dissolved in an aqueous solution, eliminating the possibility of solubility-mediated effects. This would lead one to predict that the lithium pharmacokinetics would be similar for both salts. Since that was not the case, we hypothesize that the observed “plateau effect” and modulated brain biodistribution of lithium as lithium salicylate is likely due to absorption, distribution, metabolism, and/or elimination (ADME) effects from the salicylate anion. The precise mechanism for this is unclear. However, this could be due to the chemical modification of the physiological transporter(s) of lithium ions in vivo. For example, sodium ion transporters have similar permeability for both sodium and lithium ions.21 It's feasible that salicylate chemically modifies the sodium ion transporter, changing its permeability. Future studies should confirm that coadministration of salicylic acid and FDA-approved lithium salts produce similar pharmacokinetics to those observed for lithium salicylate in this contribution.
Because lithium is so effective at treating neuropsychiatric diseases such as bipolar disorder and suicidality1,22,23 it is still used despite known toxicity issues that require frequent blood monitoring by a clinician. We argue that finding a new molecule that is a true “lithium mimetic” is probably a lost cause and recognize that crystal engineering approaches like cocrystallization could solve the toxicity issues. The preliminary data presented here demonstrates that some currently available but understudied lithium salts (e.g. lithium salicylate) may also solve the toxicity issues of conventional lithium salts (lithium carbonate and lithium citrate). However, developing new lithium salts as drugs would require significant investment from a pharmaceutical company without composition of matter patent protection. Cocrystals are patentable24 which improves the likelihood of realizing a good return on the investment required to develop them as a new drug. Moreover, cocrystals of lithium salts might also offer improved efficacy since the coformers can be rationally selected to be synergistic as discussed in recent crystal engineering efforts.13,14 Future studies should elucidate how the logical design of multi-component pharmaceutical materials can be used to improve the clinical performance of known APIs like lithium.
Both lithium salts were characterized using powder X-ray diffraction. This data is included as ESI.†
Footnotes |
† Electronic supplementary information (ESI) available: Powder X-ray diffraction of lithium salicylate and lithium lactate. See DOI: 10.1039/c3ra46962j |
‡ Present address: Center of Excellence for Aging and Brain Repair, MDC78, Department of Neurosurgery and Brain Repair, University of South Florida, College of Medicine, 33612, Tampa, FL, USA. Email: E-mail: asmith1@health.usf.edu; Fax: +1-813-974-3078; Tel: +1-813-974-1452. |
This journal is © The Royal Society of Chemistry 2014 |