Species difference on testosterone metabolism in humans and animals

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(1.National Demonstration Center for Experimental (Pharmacy) Education,Zunyi Medical University,Zunyi Guizhou 563099,China; 2.Department of Endocrinology,The Third Affiliated Hospital of Zunyi Medical University,Zunyi Guizhou 563000,China)

基礎醫學研究

Speciesdifferenceontestosteronemetabolisminhumansandanimals

HeYuqi1,ZengYao1,LingLei1,LuAnjing1,DuYimei1,YangTao1,LiBoda1,PengJie2,WuQing1

(1.National Demonstration Center for Experimental (Pharmacy) Education,Zunyi Medical University,Zunyi Guizhou 563099,China; 2.Department of Endocrinology,The Third Affiliated Hospital of Zunyi Medical University,Zunyi Guizhou 563000,China)

ObjectiveTo investigate the species difference of testosterone metabolism among human and 7 animal species including mice,rats,guinea pigs,rabbits,dogs,pigs,sheep,and cattle.MethodsTestosterone was incubated with liver microsomes of humans and investigated animals,together with CYP-mediated metabolic reaction factors including NADP Na,glucose-6-phosphate,and glucose-6-phosphate dehydrogenase.Incubation samples were introduced into HPLC to determinate testosterone and its metabolites.Heatmap was used to visualize the metabolic profile while principle component analysis was used to tell the difference of metabolic profile among species.High resolution mass spectrometry was used to assist the identification of metabolite structures.Results6β-hydroxylation is not the unique metabolic pathway of testosterone.Eighteen NADP-dependent metabolites were observed in humans or other investigated animals.Although 6β-hydroxylation showed positive correlation with the substrate elimination,this specific metabolic pathway could not compensate all substrate consumption.Guinea pig generates the most metabolites while sheep showed the weakest capacity on testosterone metabolism.Mice,dogs,and rabbits showed the most similar metabolic profile to humans.Conclusion6β-hydroxylation metabolites of testosterone was not applicable to evaluate the CYP 3A activity of all species.Mice,dogs,and rabbits are recommended in pre-clinical trial if the target is proved to be metabolized by human CYP 3A4.

testosterone; species difference; CYP 3A; metabolism

A right animal model is extremely important for drug discovery and development in particularly in pre-clinical trial stage.Thus,investigation of species difference has to be noted beforeinvivoexperiments.

Metabolic stability is an important factor leading failure of drug development[1-2].Weaker metabolic stability of drug candidate in human than in experimental animals,may lead lower exposure of the therapeutic target in drug molecules.Eventually,the significant drug effect observed in animals disappeared in humans.To prevent from misused animal model,understanding the metabolic profile and capacity of specific drug candidates in animals is necessary.The most effective and popular method to understand the metabolic capacities of animals is to check the activity of metabolic enzymes via specific probe substrates[3].

CYP 3A is the most important cytochrome P450 isozyme[4].This isozyme is responsible for metabolisms of around half market drugs.Testosterone is a famous CYP 3A4 substrate,usually was used to evaluate CYP 3A4 activity of individual human livers[5].Although testosterone 6β-hydroxylation is proved to be a selective reaction catalyzed by CYP 3A4 in humans[5],whether it’s metabolism in animals is the same or similar as human is still not clear.It means that comparison of CYP 3A activities between human and animals based on testosterone 6β-hydroxylation rate may not or may not fully reflect the real facts.

Regarding species difference on testosterone metabolism,comparison between humans and rodents has been discussed[6-7].However,in case of other normally used experimental animals such as rabbit,guinea pig,and dogs,species difference on testosterone metabolism is still not known.In special cases,some large animals,such as pigs,sheep,and cattle,are also used in pharmacology and pharmacokinetics study.Difference on testosterone metabolism between those animals and humans is also worth involving.

In the present study,testosterone metabolism in liver microsomes from human and various animal species were studied.This was the first time that species difference on testosterone metabolism was studied systematically.The metabolic profiles would indicate how similar was each investigated animals to human.The result may help to decide the right animal models in case the investigated drugs were potentially metabolized by CYP 3A.

1 Materials and Methods

1.1 Instruments and Materials An HPLC system (1100,Agilent Co.Ltd.,USA) was used to quantify testosterone,testosterone 6β-hydroxylate,and other testosterone metabolites.A Q-Exactive ultra-performance liquid chromatography coupled with high resolution mass spectrometry (UPLC-HiMS,Thermo Fisher Co.Ltd.,USA) was used to assist the identification of the structure of 6β-hydroxylated testosterone.A C18column (Zorbax C18,Agilent Co.Ltd.,USA) was used for separation.Testosterone,HPLC-grade acetonitrile and methanol,NADP Na+,glucose-6-phosphate,and glucose-6-phosphate dehydrogenase were purchased from Sigma-Aldrich (St.Louis,MO,USA).Human liver microsomes were purchased from Shanghai RILD Co.Ltd.(Shanghai,China).Mice,rats,guinea pigs,rabbits were purchased from the Animal Experimental Center of Zunyi Medical University,and livers of those animals were harvested immediately after euthanasia.Dog (beagle) livers were got from Shanghai University of Traditional Chinese Medicine.Cattle and sheep livers were harvested immediately after death of animals in food market.

1.2 Preparation of animal liver microsomes After liver homogenation,microsomes were prepared from fresh livers following our previously established method[8].All microsomes products were checked with protein concentration determination kit and diluted to the protein concentration of 10 mg/ml and stored at -80oC.

1.3 Liver microsomes incubation system for CYP-mediated metabolisms Incubation system was set following our previously published method[9].Liver microsomes (0.5 mg/ml) were pre-incubated with testosterone (50 μM),glucose-6-phosphate (1 mM),glucose-6-phosphate dehydrogenase (1 Unit/ml),and MgCl2(4 mM).Reaction start from adding of NADP Na+(1 mM).The incubation system was normalized to 100 μl with phosphate buffer saline (PBS,100 mM,pH 7.4).10 min after incubation,100 μl of acetonitrile was added to stop the reaction and precipitate proteins.All tubes were shaken thoroughly and then centrifuged at 10,000×g and 4oC for 10 min.The supernatant was used for HPLC analysis.A reaction group (n=3) involving all reaction factors,a negative group (n=3) without NADP Na+,and a blank group (n=3) without testosterone were set up.

1.4 Determination of testosterone and its metabolites To quantify metabolites of testosterone,supernatant of each incubations from 1.3 were injected to an Agilent HPLC system,eluted by Water (A) - Methanol (B) on a C18column (Agilent Zorbax C18,4.6×250 mm,5 μm).Elution gradient was shown below:0～15 min,48%～30% of A; 15～22 min,30%～20% of A;22～22.5 min,20%～5% of A,followed by a column wash program.Testosterone and its metabolites were detected at UV 254 nm.To identify which one is testosterone 6β-hydroxylate,the chromatogram of the test group was compared with that of blank and negative controls.The tentatively assigned metabolites was introduced to UPLC-HiMS to obtain the molecular weight and MS/MS spectrums.

1.5 Statistics Correlation analysis was done in SPSS 18.0 (IBM Co.Ltd.,USA) program by using the function of linear correlation (pearson).Principle component analysis and visualization at heatmap were done in R program by using mixOmics and gplots package.

2 Results

2.1 CYP-mediated metabolic profile of testosterone in humans and animals Normally,we assumed 6β-hydroxylationIn is the most common metabolic pathway of testosterone.However,in the present study,in human and animal liver microsomes incubation system,we observed 18 NADP-dependent metabolites.Metabolites were assigned with numbers based on their retention time on chromatograms (Fig1).Guinea pigs generates almost all metabolites observed in the present study,showing the highest metabolic capacity among all species investigated in the present study.Sheep generated fewest metabolites.Viewing the whole metabolic profile,obvious species difference is available.

M1～M18 are NADP-dependent metabolites of testosterone. Fig 1 Metabolic profiles of testosterone in human and animal liver microsomes incubation system specific for CYP-mediated reaction

2.2 6β-hydroxylated testosterone identification in human liver microsome incubations In total 4 peaks were observed in human liver microsome incubation system.All of the 4 peaks were just obviously observed in reaction group but not negative or blank group (Fig 2A).Based on the retention time,we assigned them as M6,M8,M16,and M18.Among the 4 metabolites,M8 showed the predominant amount.As in most publication,6β-hydroxylation is the major and specific metabolic pathway,M8 was tentatively assigned as testosterone 6β-hydroxylate.Moreover,we introduced M8 into mass spectrometer for MS2 fragment analysis.In MS2 spectrum of M8 (Fig 2B),M+H peak showed consistent value with testosterone 6β-hydroxylate.The fragments 97.07 and 109.07 were successfully aligned to structure of testosterone 6β-hydroxylate.

2.3 Correlation analysis between testosterone 6β-hydroxylation and testosterone elimination In humans and 8 animal species,generation of 6β-hydroxylated testosterone and elimination of the substrate (testosterone) were compared with each other.A significant correlation with coefficient (R) of 0.8 (0 ～ 1) was observed (Fig 3).Except for rat,guinea pig,human,and rabbit,testosterone 6β-hydroxylation and testosterone elimination generally showed the same correlation trend.In rats and guinea pigs,the spots shift away the trend line to y-axis.It means that in these two animal species,metabolic pathways other than testosterone 6β-hydroxylation play relative important role for testosterone metabolism.In contrast,in humans and rabbits,the spots shift away the trend line to x-axis,indicating the 6β-hydroxylation play the predominant role in testosterone elimination in these two animals.

A：Chromatograms of incubations from reaction,negative,and blank groups.Serial number of metabolites is the same as that shown in fig.1;B：MS2 spectrum of M8. Fig 2 Identification of the predominant metabolites in human liver microsomes

Fig 3 Correlation analysis between testosterone 6β-hydroxylation and testosterone elimination

2.4 Quantitative visualization of testosterone metabolic profile Heatmap visualization divided all metabolites into three groups (Fig 4).The first group includes M8 and M18 which were generated by all investigated species.M2,M3,M12,M17,M7,M4,M1,M10,M15,and M11 were classified into the second group located in middle part of the heatmap.These metabolites just were observed in a few species,and the amount of these metabolites are relatively low.The third class metabolites includes M6,M5,M13,M9,M16,and M14.These metabolites were not observed in all species but in most species or have relative large amount in some species.Regarding individual species,Guinea pig has the most metabolites while sheep generates the fewest metabolites.Heatmap showed clearer information for metabolic profile of testosterone than chromatograms.

From green to red represent low to high amount of metabolites. Fig 4 Quantitative visualization of metabolic profile of testosterone in humans and 7 animal species by heatmap

2.5 Principle component analysis Metabolites data matrix was inputted into R program for principle component analysis.Score plots (Fig5A) showed that metabolic profile of testosterone in guinea pigs and rats are far away from other species.However,guinea pigs shifted away to right side while rats shifted away to downside.Human generally located in the center of the score plots.Mice and rabbits located close to humans.All of these phenomenon observed in the score plots indicated that the metabolic profile of testosterone in rabbits and mice are more similar to human than other animals.Guinea pigs and rats showed so different metabolic capacities from human,however,they are also different from each other.M16,M13,and M8 got larger loading values at PC1 (Fig5B).As guinea pigs shift to right side along PC1,it means that M16,M13,and M8 were generated more in guinea pigs than in other species.With the same method,we identified that M15 and M11 have higher amount in rats than in other species.All of those findings are consistent with the directly observation on the heatmap.

Fig 5 Score plots (A) and loading plots (B) for principle component analysis of testosterone metabolisms in humans and anmials

3 Discussion

For convenience and experimental cost saving,in pharmacokinetics study,rodents and rabbits are usually selected.For sampling,in order to get the blood samples at various time points from a single animal,rats were used more frequently than mice.In some cases,large animals were also used,such as dogs,pigs,as well as sheep.Although rats were used more frequently,at genomic DNA level,rats showed different sequence of CYP 3A from humans[10-11].In rats,the enzyme with similar function with human CYP 3A4,is called as CYP 3A1[12].In mice,this enzyme is called as CYP 3A11[13].At the function level,testosterone is always used to probe the activity of CYP 3A in various animal species[14-16].However,we found that,not like usually assumed,rats showed so different metabolic profile on testosterone metabolism from humans.In contrast,testosterone metabolic profile in mice close to human more than rats.Other than rodents,in relatively large animals,rabbits and dogs showed more similar metabolic profile of testosterone to human more than sheep,cattle,and pigs.In some animal model for special disease modulation,guinea pigs were used[17].However,in the present study,regarding CYP 3A activity,guinea pigs showed so higher capacity than human and other animal species.It implies that,once a drug candidate is proved to be metabolized by CYP 3A4 in human liver microsomes or human primary hepatocyte cultures,in the pre-clinical trial stage,to make sure the same drug exposureinvivo,mice should be used in the early stage,rats and guinea pigs should be avoided.If larger animals are needed,rabbits and dogs were recommended.

Taken together,there is huge species difference in testosterone metabolism mediated by CYP3A among human and animals.Mice,dogs,rabbits are recommended to be used as animal model to promise the similar drug exposure,if a drug has already been proved to be metabolized by human CYP 3A4.

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[收稿2017-06-02;修回2017-08-10]

(編輯：譚秀榮)

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吳慶，女，本科，教授，研究方向：藥物分析技術，E-mail:68430978@qq.com；彭捷，女，本科，副主任醫師，研究方向：內分泌學，E-mail:2219282718@qq.com。

睪酮代謝的種屬差異研究

何芋岐1，曾 瑤1，凌 蕾1，陸安靜1，杜藝玫1，楊 滔1，李伯達1，彭 捷2，吳 慶1

(1.遵義醫學院 藥學國家級實驗教學示范中心(藥學實驗室)，貴州 遵義 563099；2.遵義醫學院第三附屬醫院 內分泌科，貴州 遵義 563000)

目的研究睪酮在人及小鼠、大鼠、犬、兔、豬、豚鼠、牛、羊等實驗動物肝微粒體代謝中的差異。方法睪酮分別于上述種屬肝微粒體進行孵育，加入NADP Na、葡萄糖-6-磷酸、葡萄糖-6-磷酸脫氫酶等試劑啟動體外P450酶反應。樣品采用高效液相色譜紫外檢測器及高分辨質譜進行分析。代謝輪廓用熱圖進行可視化，并用主成分分析進行差異研究。結果6β-羥基化不是睪酮唯一的代謝途徑。本研究在不同種屬中共發現了18種睪酮的代謝產物。盡管6β-羥基化在不同種屬中與睪酮的底物消除基本呈正相關，但該途徑無法補償其他途徑造成的底物消除。……