Mohammed H. Cherkaoui (1), Nicole Andenmatten (1), Rolf Fendel (2), Lydia Burgert (3,4), Chiara Fornari (5), Michael Gabel (6), Oluwaseun F. Egbelowo (7), John Ward (8), Joerg Moehrle (1), Nathalie Gobeau (1)
(1) Medicines for Malaria Venture, Geneva, Switzerland, (2) University of Tübingen, Tübingen, Germany, (3) Swiss Tropical & Public Health, Basel, Switzerland, (4) University of Basel, Basel, Switzerland, (5) AstraZeneca, Cambridge, UK, (6) University of Heidelberg, Heidelberg, Germany, (7) University of Witwatersrand, Johannesburg, South Africa, (8) University of Loughborough, Loughborough, UK
Introduction: Malaria is a deadly threat and to prevent its spread, it is important to protect people from infection, particularly from P. falciparum, probably the most prevalent and and deadly human malaria strain in Africa. Chemoprotective drugs are developed for this purpose. These should be taken at regular intervals for as long as a person is at risk. The intervals should be infrequent, ideally monthly. To reach this goal, the drug must kill the parasites not only during blood-stage but also during development in the liver, the first stage of infection after an infectious mosquito bite.
Whilst PKPD models have been developed to understand and describe the blood-stage activity of potential antimalarials, hardly any model has been proposed for liver-stage activity.
In vitro [1] and preclinical in vivo [2, 3] experiments have been developed to assess the efficacy of drugs on the liver-stage, as well as a liver-stage controlled human malaria infection model (CHMI). However, what makes it difficult to obtain a PKPD model for liver-stage activity is that unlike for blood-stage, direct measurement of liver-stage parasites is not possible.
Objective: Develop a PKPD model to describe the drug killing effect on the liver-stage parasites. The example of DSM265, a plasmodial dihydroorotate dehydrogenase (DHODH) inhibitor, is chosen to illustrate the approach.
Methods: Two liver-stage CHMIs were conducted, where healthy volunteers were treated with a DSM265 dose of 400mg administered 1, 3 or 7 days prior to infecting them with an i.v. injection of 3200. A placebo group, infected but not receiving the drug, was included in each study. Exposure of DSM265 and blood-stage parasitemia were measured in all participants [4]. Since liver-stage parasitemia could not be monitored, the information on liver-stage must be deconvoluted from the knowledge of the blood-stage activity.
A mathematical model was developed which consisted of two ordinary differential equations which described respectively the dynamic of liver-stage and blood-stage parasites over time. Each equation included a net growth rate and a drug killing rate specific to each stage. One term accounted for the release of the parasites from liver-stage to blood-stage. The parameters describing the parasite dynamics were determined from the literature [5, 6]. The drug killing rates were assumed to depend on DSM265 blood concentration according to a Hill function response. The Hill coefficient was assumed to be the same for both stages; the other parameters, Emax and EC50, were specific to the liver- and blood-stage. The blood-stage parameters were fixed to the values from a previous PKPD analysis of an induced blood-stage CHMI study. Left to be estimated: were the initial fraction of the inoculated parasites that successfully invaded hepatocytes and the liver-stage Emax (Emax,L) and EC50 (EC50,L).
The parameter estimation was conducted with Monolix (v4.3.3), an NLME modelling software. Given the limited number of subjects, some of the inter-individual variability parameters were fixed, namely those of Emax,L, EC50,L and the invader fraction Finv to 0, 0.3 and 0.5, respectively. For the validation of the model, multiple simulations of the CHMI studies were conducted and the predicted fraction of subjects with recrudescence was compared with the observations.
Results: Two estimations were performed; (i) estimation of Emax,L, and (ii) fixing Emax,L to blood-stage Emax (Emax,B). In the first estimation, a correlation between Emax,L, EC50,L is observed. As the maximum effect in the liver could not be observed, the estimation did not converge. In the second estimation, as Emax,L was fixed, the estimation converged. In both scenario, Finv was estimated at 0.2% corresponding to 6 sporozoites invading hepatocytes. In comparison, it was estimated that about 35% of the 15-120 sporozoites injected after a mosquito bite invaded hepatocytes [7]. Finally, simulations showed that the final PD model could reproduce the CHMI studies’ results, despite the limited number of volunteers.
Conclusion: In conclusion, combining knowledge of parasite dynamics, blood-stage and liver-stage CHMI studies made it possible to develop a PKPD model describing the activity of an antimalarial drug on both liver and blood stages. This will help select the appropriate dosing regimen for chemoprotection.
References:
[1]. March, S., Ng, S., Velmurugan, S., Galstian, A., Shan, J., Logan, D.J., Carpenter, A.E., Thomas, D., Sim, B.K.L., Mota, M.M., Hoffman, S.L., Bhatia, S.N.: A Microscale Human Liver Platform that Supports the Hepatic Stages of Plasmodium falciparum and vivax. Cell Host Microbe. 14, 104–115 (2013).
[2]. Vaughan, A.M., Mikolajczak, S.A., Wilson, E.M., Grompe, M., Kaushansky, A., Camargo, N., Bial, J., Ploss, A., Kappe, S.H.I.: Complete Plasmodium falciparum liver-stage development in liver-chimeric mice. J. Clin. Invest. 122, (2012).
[3]. Vaughan, A.M., Pinapati, R.S., Cheeseman, I.H., Camargo, N., Fishbaugher, M., Checkley, L.A., Nair, S., Hutyra, C.A., Nosten, F.H., Anderson, T.J.C., Ferdig, M.T., Kappe, S.H.I.: Plasmodium falciparum genetic crosses in a humanized mouse model. Nat. Methods. 12, 631–633 (2015).
[4]. Sulyok, M., Rückle, T., Roth, A., Mürbeth, R.E., Chalon, S., Kerr, N., Samec, S.S., Gobeau, N., Calle, C.L., Ibáñez, J., Sulyok, Z., Held, J., Gebru, T., Granados, P., Brückner, S., Nguetse, C., Mengue, J., Lalremruata, A., Sim, B.K.L., Hoffman, S.L., Möhrle, J.J., Kremsner, P.G., Mordmüller, B.: DSM265 for Plasmodium falciparum chemoprophylaxis: a randomised, double blinded, phase 1 trial with controlled human malaria infection. Lancet Infect. Dis. 17, 636–644 (2017).
[5]. Rankin, K.E., Graewe, S., Heussler, V.T., Stanway, R.R.: Imaging liver-stage malaria parasites. Cell. Microbiol. 12, 569–579 (2010).
[6]. Gilson, P.R., Crabb, B.S.: Morphology and kinetics of the three distinct phases of red blood cell invasion by Plasmodium falciparum merozoites. Int. J. Parasitol. 39, 91–96 (2009).
[7]. Graewe, S., Stanway, R.R., Rennenberg, A., Heussler, V.T.: Chronicle of a death foretold: Plasmodium liver stage parasites decide on the fate of the host cell. FEMS Microbiol. Rev. 36, 111–130 (2012).
Reference: PAGE 27 (2018) Abstr 8717 [www.page-meeting.org/?abstract=8717]
Poster: Methodology - New Modelling Approaches