Two notable experiments were reported in this thesis include a clinical trial where human volunteers ingested a traditional aqueous preparation of Epimedium pubescens and another female
Trang 1CHAPTER 5
CONCLUSION
5.1 Significance of findings
215
5.3 Will Epimedium exert any beneficial effects in-vivo? 223
Trang 25.1 Significance of findings
This work and the bioassay tools developed in this thesis can serve as a model framework for future projects which focus on development of estrogenic botanicals as drugs for menopause, maintenance of bone health and other conditions requiring estrogenic action in humans, from the lab bench to bedside This thesis reported the importance and value of the development and validation of appropriate tools with high throughput, in this case, were cell-based assays that were needed to rapidly track and measure global estrogenic activity in complex mixtures, from herbal preparations,
formulations to ex-vivo serum samples, which contain a myriad of bioactive compounds,
known and unknown These bioassays can also be used to rapidly screen and select compounds with optimal ERα- or ERβ-selective properties for estrogen replacement therapy that has lower adverse effects such as breast cancer cell growth, thereby reducing the number of expensive, larger-scale studies with negative clinical outcomes Although a compound or preparation is shown to be estrogenic when tested directly in the bioassays,
it cannot be assumed that it would exert similar activity when consumed
Rigorous pharmacokinetics and pharmacodynamics studies on complex botanical extracts are have been seriously constrained by the lack of suitable tools that can simultaneously track serum levels and bioactivities of the many compounds, known and unknown, that exert pharmacological effects in both animal models and humans The development of these cell-based assays enabled new insights into the pharmacokinetic and pharmacodynamic interactions underlying the biological effects of a botanical
preparation like Epimedium Two notable experiments were reported in this thesis include
a clinical trial where human volunteers ingested a traditional aqueous preparation of
Epimedium pubescens and another female ovariectomized rats being fed with an enriched
extract made from Epimedium brevicornu Although they are not directly comparable due
Trang 3to differences in their experimental design, it was found that only trace amounts of unconjugated prenylflavonoids were absorbed into the bloodstream From the rat study, it was found that the vast majority of prenylflavonoids absorbed were conjugated which were rendered relatively non-estrogenic during first pass metabolism The appearance of total amount of bioactive prenylflavonoids in sera corresponded broadly with estrogenic activity that was measured after treatment of serum samples with β-glucuronidase and sulfatase Non-linear pharmacokinetics and prolonged effects for up to 72 h following
administration of a single dose of Epimedium extract were also observed Coupled
processes of enteric and enterohepatic recycling may allow different prenylflavonoids to
be reabsorbed which resulted in longer than expected apparent plasma half-lives for some
Epimedium compounds and their conjugates β-glucuronidases present in many tissues such as bone, brain and mammary glands may convert conjugated prenylflavonoids in these tissues into their bioactive form if they become deconjugated
Gene expression profiling revealed the ability of estrogenic Epimedium
prenylflavonoids to induce the transcription of CYP1A1, which is a gene mediated by AhR AhR has been regarded as an inducible transcription factor that controls the expression of enzymes that metabolize potentially dangerous xenobiotic chemicals
However, there are AhR ligands, such as icaritin found in Epimedium, which have been
reported to be able to initiate the degradation of the ER and suppress estrogen signaling and proliferation of breast cancer cells By mediating with these pathways, these beneficial AhR ligands can be used to influence breast cancer initiation and progression
The ability of Epimedium compounds to act as dual activators for ERα and AhR mechanistically represent a new class of SERMs These compounds should be further examined for development of a new family of drugs that can be co-administered during hormone replacement therapy to reduce associated breast cancer risk
Trang 4Lastly, it is important to take note of the some experimental design pit-falls presented in this thesis which were discussed at length in Section 5.2 – Limitations Due
to a range of differences in experimental design, the results from the clinical trial where
human volunteers ingested a traditional aqueous preparation of Epimedium pubescens and another where rats were fed with an enriched extract made from Epimedium brevicornu
are best interpreted in isolation as they were not easily and cannot be directly compared Two major differences in experimental design include the use of different botanical extracts and model organisms for the two studies Notably, it is vital to ensure consistency
in the range of parameters that are to be measured and conditions that need to be employed throughout the project Lessons learnt from these two studies would greatly
enhance the design and comparison of future similar studies on Epimedium and similar
herbal drugs A good starting point for a pharmacokinetic and pharmacodynamic study on
a similar estrogenic botanical preparation would be to refer to the rat experiment reported
in this thesis which used the appropriate animal model, that is, female ovariectomized rats, which were fed separately with increasing doses of an enriched extract made from
Epimedium brevicornu and treated over a period of 72 hours, with an inclusion of a
positive control of rats fed with a standard estrogen prodrug, such as, estradiol benzoate Chemical analytical methods coupled with cell-based bioassays that can measure global estrogenicity of sera of animals were employed to understand the levels and bioactivity of conjugated and unconjugated active compounds respectively
Trang 55.2 Limitations
The first study, which involved a human clinical trial, was a pilot project to validate the cell-based bioassays in a clinical setting These bioassays were developed for use to measure global activity in serum and to evaluate the effects of ligands in complex mixtures on ERα, ERβ bioactivity and MCF-7 breast cancer cell growth In this human study, healthy male subjects, instead of females, were enrolled to reduce interference from endogenously estrogens encountered in the latter group, which may mask the effects
of administered estrogenic drugs The subjects were administered separately with
estradiol valerate or Epimedium pubescens decoction Serum samples were obtained and assayed ex-vivo for levels of estrone and estradiol by tandem mass spectrometry, for ERα and ERβ bioactivity and MCF-7 breast cancer cell proliferative effects The concentrations of icaritin and desmethylicaritin and bioactivity in sera after ingestion of
Epimedium pubescens decoction by human subjects were also measured as they are
prenylflavonoids unique to Epimedium which exerted stronger estrogenic activities than
their glycosides and flavonoids such as apigenin, kaempferol, luteolin and quercetin
In the view of the low bioactivity measured in a traditionally prepared, aqueous extract, a brand new study was done in collaboration with Dr Willmar Schwabe Pharmaceuticals (Karlsruhe, Germany) A new standardized, prenylflavonoid-enriched
extract based on Epimedium brevicornu was formulated and fed in three increasing doses
to female ovariectomized Sprague–Dawley rats, with an additional group of rats which are fed with estradiol benzoate included that served as the positive control The use of female ovariectomized Sprague–Dawley rats is a more appropriate model for testing drugs for use to enhance post-menopausal bone health
Trang 6In contrast to the earlier study, the serum bioactivity and concentrations of both unconjugated and conjugated levels of ingested prenylflavonids were monitored The levels of conjugated prenylflavonoids were included to see whether lack of estrogenic
activity of Epimedium was due to conjugation of active compounds during first pass
metabolism The quantification of prenylflavonoid glycosides, such as, icariin, icariside I
and icariside II was also undertaken to understand the metabolism of Epimedium
glycosides
Although one may want to compare whether the prenylflavonoid-enriched extract had greater bioavailability compared to the aqueous decoction due to higher amounts of compounds present, meaningful comparison is hampered by the lack of consistency in the design of the two experiments Table 22 lists these differences
Table 22: Methodological differences present in the human and rat studies
Reference Li et al., 2009 Wong et al., 2009
Model organism Human males Ovariectomized female rats
Herb species Epimedium pubescens Epimedium brevicornu
Dose(s)
Compounds
analyzed
Unconjugated
icaritin & desmethylicaritin
Unconjugated
Icariin, icariside I, icariside II, icaritin, desmethylicaritin
Total
Icariside II, icaritin, desmethylicaritin
Trang 7In the human clinical trial, male human subjects were recruited Male subjects, instead of females, were used so as to reduce interference from endogenously produced estrogens, which may mask the effects of administered estrogenic drugs With the
intention to develop Epimedium as a botanical drug for use to maintain bone health after
menopause, the more ideal candidates to use for such a study would be post-menopausal women as they will be a more relevant physiological model Differences in metabolism and plasma transport of steroids between men and women would likely to also influence the results of the study Results obtained from this human study may not be appropriate for comparison with those obtained from ovariectomized Sprague–Dawley rat study due
to species differences in the model organism used Both studies also suffered from small sample sizes as shown in Table 22 Blood at the various time-points was taken from the same human subjects over time whereas four rats were sacrificed at each time interval Inter-individual differences may affect the results of both studies more significantly due
to small sample sizes used
Further difficulties arise when different species of Epimedium and extraction
methods employed in the two studies, which result in two vastly different herbal extracts From chemical profiles obtained from quantitative analysis via chromatographic tools
used in this study, the dried residue from the aqueous decoction made from Epimedium
pubescens was found to contain 2 mg/g icariin, 0.119 mg/g icaritin and 0.031 mg/g
desmethylicaritin The amounts of icariside I and icariside II were not determined then
The enriched Epimedium brevicornu extract generally comprised more prenylflavonoids,
especially in terms of icariin but it had a lesser amount of icaritin - 143.3 mg/g icariin, 0.009 mg/g icaritin and 0.122 mg/g desmethylicaritin The amounts of icariside I and icarside II were determined to be 0.157 mg/g and 18.68 mg/g respectively The increased
Trang 8amount of prenylflavonoids in the latter extract was due to the increased solubility of the compounds in 70% ethanol that was used for extraction
Table 23: Comparison of extract dosages and amounts of prenylflavonoids ingested
from traditionally prepared aqueous Epimedium pubescens decoction and prenylflavonoid-enriched extract from Epimedium brevicornu
Preparation
Aqueous
Epimedium pubescens
decoction 1
Prenylflavonoid-enriched
Epimedium brevicornu extract
(70% ethanol extracted)
Icariside I 2 Not determined 0.0157 0.0471 0.0942
Desmethylicaritin 2 0.00279 0.0122 0.0366 0.0732
1
Decoction was prepared from 50 g of dried leaves from Epimedium pubescens Yield after extraction was
12.6% and mean weight of human subjects was 70 kg
2
Amount of extract and compounds ingested are expressed in mg / kg body weight
On a dry weight basis, the lowest dose of Epimedium brevicornu employed in the rat study (100 mg/kg) closely approximates the amount of Epimedium pubescens extract
(90.1 mg/kg) that was ingested by human subjects (Table 23) As a result of the use of different extraction methods, the profiles of prenylflavonoids ingested were markedly different, especially for icariin, which was nearly 80 times more abundant in the
prenylflavonoid-enriched Epimedium brevicornu extract (see Table 23) With reference
to the pharmacokinetic data from both studies, unconjugated desmethylicaritin was not detected in human sera in all time-points whereas it could be detected in low levels in sera
from rats that were fed with Epimedium extract at a dose of 100 mg/kg The in-vivo
Trang 9hydrolysis of glycosides like icariin can lead to the production and absorption of aglycones like icaritin and desmethylicaritin into the blood stream and this could have
occurred more significantly rats which were fed with the Epimedium brevicornu extract
due to a greater amount of icariin and similar precursor glycosides that were present As a result of such differences in starting composition of constituents, the bioavailability of
Epimedium compounds cannot be accurately ascertained from the results from both
studies
The duration of two studies and the number of metabolites that were examined also differed The rat study reported the levels of total amounts of prenylflavonoids absorbed (sum of conjugated and unconjugated metabolites) and a depot effect was observed as the duration of study was prolonged to 72 h On the other hand, the human study ended at 48 h and the total amounts of prenylflavonoids absorbed were not determined It is also not certain whether a similar depot effect would occur in humans
The results from both studies are hence best interpreted in isolation due to the differences present in their design as discussed above Lessons learnt from these two studies would greatly enhance the design and comparison of future similar studies on
Epimedium and similar herbal drugs
Trang 105.3 Will Epimedium exert any beneficial effects in-vivo?
Sera samples from human subjects who ingested the aqueous decoction contained low amounts of unconjugated icaritin and no desmethylicaritin The dose was likely to be too low and any absorbed aglycones would have been conjugated during extensive first pass metabolism Any unconjugated compounds would be present in trace levels and become bound tightly to albumin in sera during circulation This could be the reason why estrogenicity of sera was not detected in human serum samples as levels of unconjugated prenylflavonoids that were present in nanomolar quantities that are well below the detection limits of the panel of cell-based bioassays (see dose-response curves in Fig 15)
Although the amount of prenylflavonoids was enriched in the alcoholic
Epimedium brevicornu extract that was fed in three increasing doses in the rat study, due
to extensive first pass metabolism, the peak serum levels of both unconjugated aglycones, namely, icaritin and desmethylicaritin, as well as, two estrogenic monoglucosides, icariside I and icariside II, were in the low nanomolar levels, even at the highest dose of
Epimedium pubescens extract in rats The estrogenicity of rat serum samples for 100 and
200 mg/kg dosages were also not significant when measured the ERα cell-based bioassay Interestingly, estrogenicity equivalent to 10 pM of estradiol was detected in sera at the 8 h
time-point from rats fed with the highest dose of Epimedium brevicornu extract (600
mg/kg dose) (Fig 39) which was likely to be conferred by estrogenic metabolites not
studied in this work This is indicative that Epimedium can exert estrogenicity in-vivo and
future work entails optimization of the human equivalent dosage of extract that need to be taken to ensure efficacy
Although the results for a single dose of Epimedium extract in this study may imply that the preparations would not exert significant estrogenicity in-vivo due to the low