Orally Bioavailable Endochin-like Quinolone Carbonate Ester Prodrug Reduces Toxoplasma gondii Brain Cysts

J. Stone Doggett, Tracey Schultz, Alyssa J. Miller, Igor Bruzual, Sovitj Pou, Rolf Winter, Rozalia Dodean, Lev N. Zakharov, Aaron Nilsen, Michael K Riscoe, Vern B Carruthers

Preprint posted on 20 March 2020

Article now published in Antimicrobial Agents and Chemotherapy at

The quinolone in the ester mask: Doggett et al. adopt a prodrug approach to reduce Toxoplasma gondii brain cysts

Selected by Zhang-He Goh

Background of preprint

Toxoplasma gondii is a widespread parasite that can be especially dangerous in immunocompromised patients. Unfortunately, the few existing treatments that are effective against T. gondii also have high rates of adverse events that lead to discontinuation in 30% of patients.

About a decade earlier, endochin-like quinolones (ELQ) were found to inhibit apicomplexan pathogens, including T. gondii [1] and Plasmodium falciparum [2]. Doggett et al. had previously identified ELQ-271 and ELQ-316 (Fig. 1) as potential compounds against T. gondii, and specifically highlighted ELQ-316 as a prospective lead due to its “efficacy and specificity for the T. gondii cytochrome b over the human cytochrome b” in their 2012 paper [1]. However, owing to pharmacokinetic issues, the oral dosing of ELQ-316 was not effective.

Figure 1. ELQs discussed in Doggett et al.’s previous work and current preprint

In their preprint, Doggett et al. synthesised and characterised ELQ-334 (Fig. 1), a carbonate ester of ELQ-316, using the carbonate ester-masking approach to increase bioavailability. The preprint authors characterised the pharmacokinetic parameters and efficacy of ELQ-334 against both acute and latent toxoplasmosis.

Key findings of preprint

Doggett et al. administered ELQ-316 and ELQ-334 to mice orally and their pharmacokinetic parameters were characterised in the plasma and brain: cmax­, Tmax, AUC0-96, and t1/2 (Table 1).

Table 1. Pharmacokinetic parameters of ELQ-316 from ELQ-316 and ELQ-334.

The preprint authors also found that the mean plasma concentration of ELQ-334 did not exceed 238 ng/ml. Taken together, these findings led the preprint authors to conclude that the prodrug ELQ-334 was rapidly converted to the active compound ELQ-316.

By testing the efficacy of ELQ-334 against acute toxoplasmosis in mice, Doggett et al. found that the administration of ELQ-334 significantly boosted survivability of the mice. The authors also tested ELQ-334 in latent T. gondii infection and found that ELQ-334 significantly reduced the number of T. gondii cysts compared to control mice.

What I like about this preprint

I selected this preprint because it illustrates an important tenet that underlies the work done by researchers and healthcare professionals: for a drug to be effective, it needs to reach effective concentrations at the site of action. For decades, the brain has presented a challenge to anyone working in the healthcare sector. Formulators and medicinal chemists try different strategies to help drugs penetrate the Blood-Brain Barrier (BBB); while pharmacists and physicians carefully consider the pharmacokinetics of a drug in relation to the site of infection before prescribing or dispensing it to patients.

Events in recent months have once again underscored the need for research into combating infectious diseases. As the global population—especially the subset residing in the developed world—continues to age, patients are becoming increasingly complex and vulnerable. The twin threats of an ageing population and growing antimicrobial resistance will fuel the urgency for more potent antimicrobials in this decade. At the same time, these antimicrobials also need to be sufficiently selective to inflict minimal side effects on their hosts.

From this perspective, the findings that Doggett et al. have highlighted in this preprint will impact many of these considerations. The first is that ELQ-316 was selected over ELQ-271 for optimisation, owing to ELQ-316’s higher potency and better selectivity [1]. Then there is the challenge to optimise the absorption pharmacokinetics of ELQ-316, which the preprint authors resolved by creating ELQ-334, a prodrug carbonate ester of ELQ-316. Finally, the preprint authors also explored the role that ELQ-334 might play in eradicating latent infections, a consideration that will become increasingly relevant as we further develop healthcare capabilities to extend the human lifespan and support medically vulnerable populations.

Questions for authors

  • In the noncompartmental pharmacokinetic analysis of ELQ-316 and ELQ-334, you compared the brain concentrations of ELQ-316 derived from ELQ-316 and ELQ-334. In lines 124 – 125 of the preprint, brain concentrations for ELQ-316 derived from ELQ-316 were reported in ng/g, while in lines 122 – 123 of the preprint, the brain concentrations for ELQ-316 derived from ELQ-334 were reported in ng/ml. In 3B of the preprint, was it assumed that the density of brain tissue is 1 g/mL, so that this would allow us to do a direct comparison between the two results?
  • What were the AUC0-96 and the t1/2 of ELQ-316 derived from ELQ-316? Were there significant differences between the t1/2 of ELQ-316 derived from ELQ-316 and the t1/2 of ELQ-316 derived from ELQ-334 ( 3A of the preprint)?
  • The hypothesis that the conversion of ELQ-334 to ELQ-316 may “be caused in part by esterases in the gastrointestinal tract” (preprint lines 127 – 129) is an interesting one, given that in preprint lines 72 – 73, the decrease in the bioavailability of ELQs was attributed to the precipitation in the gastrointestinal tract. Therefore, it appears to follow that, were the ester cleavage performed in the gastrointestinal tract, the same problems would arise from an absorption perspective. Would it be possible that it is the ubiquitous body esterases—such as those in the liver—that are instead responsible for the first-pass effect?


[1] Doggett JS, Nilsen A, Forquer I, Wegmann KW, Jones-Brando L, Yolken RH, Bordón C, Charman SA, Katneni K, Schultz T, Burrows JN, Hinrichs DJ, Meunier B, Carruthers VB, Riscoe MK, Endochin-like quinolones are highly efficacious against acute and latent experimental toxoplasmosis, Proc Natl Acad Sci U S A 109(39) (2012) 15936-15941.

[2] Winter R, Kelly JX, Smilkstein MJ, Hinrichs D, Koop DR, Riscoe MK, Optimization of endochin-like quinolones for antimalarial activity, Exp Parasitol 127(2) (2011) 545-551.

Tags: #toxoplasmosis, antimalarial, endochin-like quinolone, pharmacokinetics, prodrug, toxoplasma gondii

Posted on: 10 April 2020


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