Chemistry:Estrogen ester

Short description: Ester of an estrogen

Not to be confused with Esterified estrogens or Conjugated estrogens.

An estrogen ester is an ester of an estrogen, most typically of estradiol but also of other estrogens such as estrone, estriol, and even nonsteroidal estrogens like diethylstilbestrol. Esterification renders estradiol into a prodrug of estradiol with increased resistance to first-pass metabolism, slightly improving its oral bioavailability. In addition, estrogen esters have increased lipophilicity, which results in a longer duration when given by intramuscular or subcutaneous injection due to the formation of a long-lasting local depot in muscle and fat. Conversely, this is not the case with intravenous injection or oral administration. Estrogen esters are rapidly hydrolyzed into their parent estrogen by esterases once they have been released from the depot. Because estradiol esters are prodrugs of estradiol, they are considered to be natural and bioidentical forms of estrogen.

Medical uses

Estrogen esters are used in hormone therapy, hormonal contraception, and high-dose estrogen therapy (e.g., for prostate cancer and breast cancer), among other indications. The first estrogen ester to be marketed was estradiol benzoate in 1933, which was followed by many more. One of the most widely used estradiol esters is estradiol valerate, which was first introduced in 1954. Other major estradiol esters that are or have been used in medicine include estradiol acetate, estradiol cypionate, estradiol dipropionate, estradiol enantate, estradiol undecylate, and polyestradiol phosphate (an estrogen ester polymer), as well as the nitrogen mustard alkylating antineoplastic agent estramustine phosphate (estradiol normustine phosphate).

The most common vehicles for injections of steroids and steroid esters are oil solutions, but aqueous solutions, aqueous suspensions, and emulsions have also been used. The durations of estrogen esters are not prolonged if they are given orally, vaginally, or by intravenous injection.

Pharmacology

Estrogen esters are essentially inactive themselves, with esters such as estradiol valerate and estradiol sulfate having about 2% of the affinity of estradiol for the estrogen receptor. Likewise, the estrogen ether mestranol (ethinylestradiol 3-methyl ether) has about 1% of the affinity of estradiol for the estrogen receptor. Estrone sulfate has less than 1% of the affinity of estradiol for the estrogen receptor. As such, estrogen esters do not bind to the estrogen receptor except at extremely high concentrations. The residual affinity of estrogen esters for the estrogen receptor in bioassays may actually be due to conversion into the parent estrogen, as attempts to prevent or limit this conversion have been found to abolish binding to the estrogen receptor and estrogenicity.

Affinities of estrogen receptor ligands for the ERα and ERβ

Ligand	Other names	Relative binding affinities (RBA, %)a		Absolute binding affinities (Ki, nM)a		Action
		ERα	ERβ	ERα	ERβ
Estradiol	E2; 17β-Estradiol	100	100	(0.04–0.24)	(0.10–2.08)	Estrogen
Estrone	E1; 17-Ketoestradiol	(0.7–60)	(1.36–52)	(0.3–1.01)	(0.35–9.24)	Estrogen
Estriol	E3; 16α-OH-17β-E2	(4.03–56)	(14.0–44.6)	(0.35–1.4)	(0.63–0.7)	Estrogen
Estetrol	E4; 15α,16α-Di-OH-17β-E2	4.0	3.0	4.9	19	Estrogen
Alfatradiol	17α-Estradiol	(7–80.1)	(2–42)	0.2–0.52	0.43–1.2	Metabolite
16-Epiestriol	16β-Hydroxy-17β-estradiol	(4.94–63)	50	?	?	Metabolite
17-Epiestriol	16α-Hydroxy-17α-estradiol	(29–103)	79–80	?	?	Metabolite
16,17-Epiestriol	16β-Hydroxy-17α-estradiol	1.0	13	?	?	Metabolite
2-Hydroxyestradiol	2-OH-E2	(7–81)	11–35	2.5	1.3	Metabolite
2-Methoxyestradiol	2-MeO-E2	0.0027–2.0	1.0	?	?	Metabolite
4-Hydroxyestradiol	4-OH-E2	(8–70)	7–56	1.0	1.9	Metabolite
4-Methoxyestradiol	4-MeO-E2	2.0	1.0	?	?	Metabolite
2-Hydroxyestrone	2-OH-E1	2.0–4.0	0.2–0.4	?	?	Metabolite
2-Methoxyestrone	2-MeO-E1	<0.001–<1	<1	?	?	Metabolite
4-Hydroxyestrone	4-OH-E1	1.0–2.0	1.0	?	?	Metabolite
4-Methoxyestrone	4-MeO-E1	<1	<1	?	?	Metabolite
16α-Hydroxyestrone	16α-OH-E1; 17-Ketoestriol	2.0–6.5	35	?	?	Metabolite
2-Hydroxyestriol	2-OH-E3	2.0	1.0	?	?	Metabolite
4-Methoxyestriol	4-MeO-E3	1.0	1.0	?	?	Metabolite
Estradiol sulfate	E2S; Estradiol 3-sulfate	<1	<1	?	?	Metabolite
Estradiol disulfate	Estradiol 3,17β-disulfate	0.0004	?	?	?	Metabolite
Estradiol 3-glucuronide	E2-3G	0.0079	?	?	?	Metabolite
Estradiol 17β-glucuronide	E2-17G	0.0015	?	?	?	Metabolite
Estradiol 3-gluc. 17β-sulfate	E2-3G-17S	0.0001	?	?	?	Metabolite
Estrone sulfate	E1S; Estrone 3-sulfate	<1	<1	>10	>10	Metabolite
Estradiol benzoate	EB; Estradiol 3-benzoate	10	?	?	?	Estrogen
Estradiol 17β-benzoate	E2-17B	11.3	32.6	?	?	Estrogen
Estrone methyl ether	Estrone 3-methyl ether	0.145	?	?	?	Estrogen
ent-Estradiol	1-Estradiol	1.31–12.34	9.44–80.07	?	?	Estrogen
Equilin	7-Dehydroestrone	(4.0–28.9)	13.0–49	0.79	0.36	Estrogen
Equilenin	6,8-Didehydroestrone	2.0–15	7.0–20	0.64	0.62	Estrogen
17β-Dihydroequilin	7-Dehydro-17β-estradiol	7.9–113	7.9–108	0.09	0.17	Estrogen
17α-Dihydroequilin	7-Dehydro-17α-estradiol	(18–41)	14–32	0.24	0.57	Estrogen
17β-Dihydroequilenin	6,8-Didehydro-17β-estradiol	35–68	90–100	0.15	0.20	Estrogen
17α-Dihydroequilenin	6,8-Didehydro-17α-estradiol	20	49	0.50	0.37	Estrogen
Δ8-Estradiol	8,9-Dehydro-17β-estradiol	68	72	0.15	0.25	Estrogen
Δ8-Estrone	8,9-Dehydroestrone	19	32	0.52	0.57	Estrogen
Ethinylestradiol	EE; 17α-Ethynyl-17β-E2	(68.8–480)	(2.0–144)	0.02–0.05	0.29–0.81	Estrogen
Mestranol	EE 3-methyl ether	?	2.5	?	?	Estrogen
Moxestrol	RU-2858; 11β-Methoxy-EE	35–43	5–20	0.5	2.6	Estrogen
Methylestradiol	17α-Methyl-17β-estradiol	70	44	?	?	Estrogen
Diethylstilbestrol	DES; Stilbestrol	(89.1–468)	(61.2–295)	0.04	0.05	Estrogen
Hexestrol	Dihydrodiethylstilbestrol	(31–302)	60–234	0.06	0.06	Estrogen
Dienestrol	Dehydrostilbestrol	(20.4–223)	56–404	0.05	0.03	Estrogen
Benzestrol (B2)	–	114	?	?	?	Estrogen
Chlorotrianisene	TACE	1.74	?	15.30	?	Estrogen
Triphenylethylene	TPE	0.074	?	?	?	Estrogen
Triphenylbromoethylene	TPBE	2.69	?	?	?	Estrogen
Tamoxifen	ICI-46,474	(0.1–47)	(0.28–6)	3.4–9.69	2.5	SERM
Afimoxifene	4-Hydroxytamoxifen; 4-OHT	(1.7–257)	(0.98–339)	(0.1–3.61)	0.04–4.8	SERM
Toremifene	4-Chlorotamoxifen; 4-CT	?	?	7.14–20.3	15.4	SERM
Clomifene	MRL-41	(19.2–37.2)	12	0.9	1.2	SERM
Cyclofenil	F-6066; Sexovid	151–152	243	?	?	SERM
Nafoxidine	U-11,000A	30.9–44	16	0.3	0.8	SERM
Raloxifene	–	(7.8–69)	(0.54–16)	0.188–0.52	20.2	SERM
Arzoxifene	LY-353,381	?	?	0.179	?	SERM
Lasofoxifene	CP-336,156	10.2–166	19.0	0.229	?	SERM
Ormeloxifene	Centchroman	?	?	0.313	?	SERM
Levormeloxifene	6720-CDRI; NNC-460,020	1.55	1.88	?	?	SERM
Ospemifene	Deaminohydroxytoremifene	2.63	1.22	?	?	SERM
Bazedoxifene	–	?	?	0.053	?	SERM
Etacstil	GW-5638	4.30	11.5	?	?	SERM
ICI-164,384	–	(3.70–97.7)	166	0.2	0.08	Antiestrogen
Fulvestrant	ICI-182,780	(9.4–325)	(2.05–40.5)	0.42	1.3	Antiestrogen
Propylpyrazoletriol	PPT	(10.0–89.1)	0.12	0.40	92.8	ERα agonist
16α-LE2	16α-Lactone-17β-estradiol	14.6–57	0.089	0.27	131	ERα agonist
16α-Iodo-E2	16α-Iodo-17β-estradiol	30.2	2.30	?	?	ERα agonist
Methylpiperidinopyrazole	MPP	11	0.05	?	?	ERα antagonist
Diarylpropionitrile	DPN	0.12–0.25	6.6–18	32.4	1.7	ERβ agonist
8β-VE2	8β-Vinyl-17β-estradiol	0.35	22.0–83	12.9	0.50	ERβ agonist
Prinaberel	ERB-041; WAY-202,041	0.27	67–72	?	?	ERβ agonist
ERB-196	WAY-202,196	?	180	?	?	ERβ agonist
Erteberel	SERBA-1; LY-500,307	?	?	2.68	0.19	ERβ agonist
SERBA-2	–	?	?	14.5	1.54	ERβ agonist
Coumestrol	–	(0.0117–94)	(0.41–185)	0.14–80.0	0.07–27.0	Xenoestrogen
Genistein	–	(0.0012–16)	(0.86–87)	2.6–126	0.3–12.8	Xenoestrogen
Equol	–	0.2–0.287	(0.10–2.85)	?	?	Xenoestrogen
Daidzein	–	(0.0018–9.3)	(0.04–17.1)	2.0	85.3	Xenoestrogen
Biochanin A	–	(0.022–0.15)	(0.010–1.2)	174	8.9	Xenoestrogen
Kaempferol	–	(0.029–0.10)	(0.002–3.00)	?	?	Xenoestrogen
Naringenin	–	(<0.001–0.01)	(0.11–0.33)	?	?	Xenoestrogen
8-Prenylnaringenin	8-PN	4.4	?	?	?	Xenoestrogen
Quercetin	–	<0.001–0.01	0.002–0.040	?	?	Xenoestrogen
Ipriflavone	–	<0.01	<0.01	?	?	Xenoestrogen
Miroestrol	–	0.39	?	?	?	Xenoestrogen
Deoxymiroestrol	–	2.0	?	?	?	Xenoestrogen
β-Sitosterol	–	<0.001–0.0875	<0.001–0.016	?	?	Xenoestrogen
Resveratrol	–	<0.001–0.0032	?	?	?	Xenoestrogen
α-Zearalenol	–	(13–52.5)	?	?	?	Xenoestrogen
β-Zearalenol	–	(0.032–13)	?	?	?	Xenoestrogen
Zeranol	α-Zearalanol	48–111	?	?	?	Xenoestrogen
Taleranol	β-Zearalanol	(13–17.8)	14	0.8	0.9	Xenoestrogen
Zearalenone	ZEN	(2.04–28)	(2.43–31.5)	?	?	Xenoestrogen
Zearalanone	ZAN	0.51	?	?	?	Xenoestrogen
Bisphenol A	BPA	(0.008–1.0)	(0.002–4.23)	195	35	Xenoestrogen
Endosulfan	EDS	<0.001–<0.01	<0.01	?	?	Xenoestrogen
Kepone	Chlordecone	0.0069–0.2	?	?	?	Xenoestrogen
o,p'-DDT	–	0.0073–0.4	?	?	?	Xenoestrogen
p,p'-DDT	–		?	?	?	Xenoestrogen
Methoxychlor	p,p'-Dimethoxy-DDT	(<0.001–0.02)	0.01–0.13	?	?	Xenoestrogen
HPTE	Hydroxychlor; p,p'-OH-DDT	1.2–1.7	?	?	?	Xenoestrogen
Testosterone	T; 4-Androstenolone	<0.0001–<0.01	<0.002–0.040	>5000	>5000	Androgen
Dihydrotestosterone	DHT; 5α-Androstanolone	(<0.001–0.05)	0.0059–0.17	221–>5000	73–1688	Androgen
Nandrolone	19-Nortestosterone; 19-NT	0.01	0.23	765	53	Androgen
Dehydroepiandrosterone	DHEA; Prasterone	(<0.001–0.04)	0.019–0.07	245–1053	163–515	Androgen
5-Androstenediol	A5; Androstenediol	6	17	3.6	0.9	Androgen
4-Androstenediol	–	0.5	0.6	23	19	Androgen
4-Androstenedione	A4; Androstenedione	<0.01	<0.01	>10000	>10000	Androgen
3α-Androstanediol	3α-Adiol	0.07	0.3	260	48	Androgen
3β-Androstanediol	3β-Adiol	3	7	6	2	Androgen
Androstanedione	5α-Androstanedione	<0.01	<0.01	>10000	>10000	Androgen
Etiocholanedione	5β-Androstanedione	<0.01	<0.01	>10000	>10000	Androgen
Methyltestosterone	17α-Methyltestosterone	<0.0001	?	?	?	Androgen
Ethinyl-3α-androstanediol	17α-Ethynyl-3α-adiol	4.0	<0.07	?	?	Estrogen
Ethinyl-3β-androstanediol	17α-Ethynyl-3β-adiol	50	5.6	?	?	Estrogen
Progesterone	P4; 4-Pregnenedione	<0.001–0.6	<0.001–0.010	?	?	Progestogen
Norethisterone	NET; 17α-Ethynyl-19-NT	(0.0015–<0.1)	(0.01–0.3)	152	1084	Progestogen
Norethynodrel	5(10)-Norethisterone	(0.3–0.7)	<0.1–0.22	14	53	Progestogen
Tibolone	7α-Methylnorethynodrel	(0.45–2.0)	0.2–0.076	?	?	Progestogen
Δ4-Tibolone	7α-Methylnorethisterone	0.069–<0.1	0.027–<0.1	?	?	Progestogen
3α-Hydroxytibolone	–	(1.06–5.0)	0.6–0.8	?	?	Progestogen
3β-Hydroxytibolone	–	(0.75–1.9)	0.070–0.1	?	?	Progestogen
Footnotes: a = (1) Binding affinity values are of the format "median (range)" (# (#–#)), "range" (#–#), or "value" (#) depending on the values available. The full sets of values within the ranges can be found in the Wiki code. (2) Binding affinities were determined via displacement studies in a variety of in-vitro systems with labeled estradiol and human ERα and ERβ proteins (except the ERβ values from Kuiper et al. (1997), which are rat ERβ). Sources: See template page.

In general, the longer the fatty acid ester chain of an estrogen ester, the greater its lipophilicity, and the longer the duration of the estrogen ester with intramuscular injection. It has been said that, via intramuscular injection, the duration of estradiol benzoate (with an ester of length 1 carbon plus a benzene ring) is 2 to 3 days, of estradiol dipropionate (with two esters each of length 2 carbons) is 1 to 2 weeks, of estradiol valerate (ester of 5 carbons) is 1 to 3 weeks, and of estradiol cypionate (ester of 3 carbons plus a cyclopentane ring) is 3 to 4 weeks. Estradiol enantate (ester of 7 carbons) has a duration of around 20 days. Likewise, estradiol undecylate (ester of 10 carbons) has a very extended duration, which is longer than that of all of the aforementioned esters.

Pharmacokinetics of three estradiol esters by intramuscular injection

Estrogen	Dose	Peak levels	Time to peak	Duration
Estradiol benzoate	5 mg	E2: 940 pg/mL E1: 343 pg/mL	E2: 1.8 days E1: 2.4 days	4–5 days
Estradiol valerate	5 mg	E2: 667 pg/mL E1: 324 pg/mL	E2: 2.2 days E1: 2.7 days	7–8 days
Estradiol cypionate	5 mg	E2: 338 pg/mL E1: 145 pg/mL	E2: 3.9 days E1: 5.1 days	11 days
Notes: All via i.m. injection of oil solution. Determinations via radioimmunoassay with chromatographic separation. Sources: See template.

Polyestradiol phosphate is an atypical estradiol ester. It is a phosphoric acid ester of estradiol in the form of a polymer, with an average polymer chain length of approximately 13 repeat units of estradiol phosphate. It is slowly cleaved into estradiol and phosphoric acid by phosphatases. Compared to conventional estradiol esters, polyestradiol phosphate has an extremely long duration; its elimination half-life is approximately 70 days. Whereas conventional estradiol esters form a long-lasting depot in muscle and fat at the site of injection, this is not the case with polyestradiol phosphate. Instead, polyestradiol phosphate is taken up rapidly into the bloodstream following injection (by 90% within 24 hours), where it circulates, and is accumulated in the reticuloendothelial system. Unlike other estradiol esters, polyestradiol phosphate is resistant to hydrolysis, which may be because it is a phosphatase inhibitor and may inhibit its own metabolism.

Estrogen esters also occur naturally in the body, for instance estrogen conjugates like estrone sulfate and estrone glucuronide and the very long-lived lipoidal estradiol, which is constituted by ultra-long-chain esters like estradiol palmitate (ester of 16 carbons) and estradiol stearate (ester of 18 carbons).

Chemistry

Estradiol plus the fatty acid valeric acid (valerate) equals estradiol valerate, a C17β ester of estradiol and one of the most widely used estrogen esters.

Polyestradiol phosphate, a polymer of estradiol phosphate, the C17β phosphoric acid ester of estradiol. It has on average of 13 repeat units.

Estradiol esters have an ester moiety, usually a straight-chain fatty acid (e.g., valeric acid) or an aromatic fatty acid (e.g., benzoic acid), attached at the C3 and/or C17β positions of the steroid nucleus. These alkoxy moieties are substituted in place of the hydroxyl groups present in the unesterified estradiol molecule. Fatty acid esters serve to increase the lipophilicity of estradiol, increasing its solubility in fat. This causes them to form a depot with intramuscular or subcutaneous injection and gives them a long duration when administered by these routes.

Some estradiol esters have other moieties instead of fatty acids as the esters. Such esters include sulfuric acid (as in estradiol sulfate), sulfamic acid (as in estradiol sulfamate), phosphoric acid (as in estradiol phosphate), glucuronic acid (as in estradiol glucuronide, and others (e.g., estramustine phosphate (estradiol 3-normustine 17β-phosphate)). These esters are all hydrophilic, and have greater water solubility than estradiol or fatty acid estradiol esters. Unlike fatty acid estradiol esters, water-soluble estradiol esters can be administered by intravenous injection.

A few estrogen esters are polymers. These include polyestradiol phosphate and polyestriol phosphate, which are polymers of estradiol phosphate and estriol phosphate monomers, respectively. The monomers are connected in both cases by phosphate groups via the C3 and C17β positions. Polyestradiol phosphate has an average polymer chain length of approximately 13 repeat units of estradiol phosphate. That is, each polyestradiol phosphate molecule is a polymer consisting on average of 13 estradiol phosphate molecules bonded together. These polymeric estrogen esters are hydrophilic and water-soluble. Upon intramuscular injection, they do not form a depot and instead are rapidly absorbed into the circulation. However, they are only slowly cleaved into monomers, and as a result, have a very long duration in the body even outlasting that of many longer-chain fatty-acid estrogen esters.

Chemical structures of estradiol and major estradiol esters

• 
  Estradiol
• 
  Estradiol acetate
• 
  Estradiol benzoate
• 
  Estradiol dipropionate
• 
  Estradiol valerate
• 
  Estradiol cypionate
• 
  Estradiol enantate
• 
  Estradiol undecylate
• 
  Polyestradiol phosphate

Structural properties of selected estradiol esters

Estrogen	Structure	Ester(s)				Relative mol. weight	Relative E2 contentb	logPc
		Position(s)	Moiet(ies)	Type	Lengtha
Estradiol		–	–	–	–	1.00	1.00	4.0
Estradiol acetate		C3	Ethanoic acid	Straight-chain fatty acid	2	1.15	0.87	4.2
Estradiol benzoate		C3	Benzenecarboxylic acid	Aromatic fatty acid	– (~4–5)	1.38	0.72	4.7
Estradiol dipropionate		C3, C17β	Propanoic acid (×2)	Straight-chain fatty acid	3 (×2)	1.41	0.71	4.9
Estradiol valerate		C17β	Pentanoic acid	Straight-chain fatty acid	5	1.31	0.76	5.6–6.3
Estradiol benzoate butyrate		C3, C17β	Benzoic acid, butyric acid	Mixed fatty acid	– (~6, 2)	1.64	0.61	6.3
Estradiol cypionate		C17β	Cyclopentylpropanoic acid	Aromatic fatty acid	– (~6)	1.46	0.69	6.9
Estradiol enanthate		C17β	Heptanoic acid	Straight-chain fatty acid	7	1.41	0.71	6.7–7.3
Estradiol dienanthate		C3, C17β	Heptanoic acid (×2)	Straight-chain fatty acid	7 (×2)	1.82	0.55	8.1–10.4
Estradiol undecylate		C17β	Undecanoic acid	Straight-chain fatty acid	11	1.62	0.62	9.2–9.8
Estradiol stearate		C17β	Octadecanoic acid	Straight-chain fatty acid	18	1.98	0.51	12.2–12.4
Estradiol distearate		C3, C17β	Octadecanoic acid (×2)	Straight-chain fatty acid	18 (×2)	2.96	0.34	20.2
Estradiol sulfate		C3	Sulfuric acid	Water-soluble conjugate	–	1.29	0.77	0.3–3.8
Estradiol glucuronide		C17β	Glucuronic acid	Water-soluble conjugate	–	1.65	0.61	2.1–2.7
Estramustine phosphated		C3, C17β	Normustine, phosphoric acid	Water-soluble conjugate	–	1.91	0.52	2.9–5.0
Polyestradiol phosphatee		C3–C17β	Phosphoric acid	Water-soluble conjugate	–	1.23f	0.81f	2.9g
Footnotes: a = Length of ester in carbon atoms for straight-chain fatty acids or approximate length of ester in carbon atoms for aromatic fatty acids. b = Relative estradiol content by weight (i.e., relative estrogenic potency). c = Experimental or predicted octanol/water partition coefficient (i.e., lipophilicity/[[hydrophobicity]]). Retrieved from PubChem, ChemSpider, and DrugBank. d = Also known as estradiol normustine phosphate. e = Polymer of estradiol phosphate (~13 repeat units). f = Relative molecular weight or estradiol content per repeat unit. g = logP of repeat unit (i.e., estradiol phosphate). Sources: See individual articles.

See also

• Pharmacokinetics of estradiol
• List of estrogen esters
• List of estrogens
• Steroid ester
• Progestogen ester
• Androgen ester

Further reading

• "Longacting steroid preparations". Acta Clin Belg 30 (1): 48–55. 1975. doi:10.1080/17843286.1975.11716973. PMID 1231448. 
• "A comparison of the pharmacokinetic properties of three estradiol esters". Contraception 21 (4): 415–24. 1980. doi:10.1016/s0010-7824(80)80018-7. PMID 7389356. 
• "Pharmacokinetic and pharmacological features of oestradiol valerate". Maturitas 4 (4): 315–24. 1982. doi:10.1016/0378-5122(82)90064-0. PMID 7169965. 
• "Pharmacodynamic effects of once-a-month combined injectable contraceptives". Contraception 49 (4): 361–85. 1994. doi:10.1016/0010-7824(94)90033-7. PMID 8013220.