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Bioavailability of norethindrone and menstrual pattern after insertion of norethindrone acetate implants in rhesus monkeys
CONTRACEPTION BIOAVAILABILITY OF NORETHINDRONE AND MENSTRUAL PATTERN AFTER INSERTION OF NORETHINDRONE ACETATE IMPLANTS IN RHESUS MONKEYS
P.Jha, K.Murugesan and A. Farooq Department of Reproductive Biology All India Institute of Medical Sciences New Delhi - 110029, INDIA
ABSTRACT Subdermal silastic implants releasing norethindrone acetate were inserted in five rhesus monkeys for a period of seven months. The serum norethindrone (NET) levels were estimated from blood samples collected fortnightly. There was an initial high level of NET followed by fluctuating levels of NET in 4 out of five monkeys. The serum NET values showed individual variation between time periods as well as between monkeys. The analysis of menstrual cycles showed initial amenorrhoea up to day 120-140 in four out of five monkeys. Only one monkey showed vaginal bleeding pattern comparable to normal menstrual cycles; this monkey had serum NET levels less than 10 ng/ml. The phannacokinetic and pharmacodynamic parameters were similar to those reported in the human female.
INTRODUCTION Long-acting hormonal preparations such as injectables and subdermal silastic devices have been tested in humans (l-4) for fertility regulation and found to be effective from a few months to a few years. As these hormonal contraceptive preparations are long-acting, they are considered as methods of choice for fertility regulation in developing countries where motivation is poor and follow-up is difficult. The calculable microquantity of contraceptive steroids released from the subdermal silastic implants has a number of advantages over the oral or injectable preparations where the woman is exposed to very high doses of contraceptive steroids. Though these subdermal silastic implants were considered to produce lesser side effects, yet menstrual disturbances during the first few cycles were observed in many studies (l-4). The norethindrone acetate (NETA)releasing subdennal implant was tested and found to be effective for a period up to 6 months in humans (5). But, it was shown to have varying menstrual problems. Belsey and associates (6) have studied the menstrual bleeding patterns among contraceptive users and reported varying menstrual patterns with combination oral contraceptives, progestogen only and injectable regimens. The present study was carried out in rhesus monkeys which have menstrual pattern similar to the human female in order to understand the bioavailability of NET after the insertion of NETreleasing subdermal silastic implants and to ascertain whether the Submitted for publication June 18, 1990 Accepted for publication July 17, 1991
SEPTEMBER 1991 VOL. 44 NO. 3
327
CONTRACEPTION circulating NET levels had any correlation with the observed menstrual pattern. The observations made in this study have potential value in developing another useful long-acting contraceptive modality. MATERIALS AND METHODS Implants: The implants were prepared by filling 40 mg NETA in a carefully washed, dried and inspected silastic tubing (Dow-Corning Corporation, Midland, Michigan) of 22 mm length and 0.59 mm wall thickness (7). Animals: Five adult healthy female rhesus monkeys weighing between 4.5-5.5 kg, maintained at the Primate Research Facility of AIIMS were used for this study. Monkeys with at least two consecutive menstrual cycles of normal duration were selected for the study. They were kept in a single room and were given the same diet. Monkeys were inserted subdennally with silastic implants releasing NETA on the lateral side of the thigh under general anaesthesia between day 4 to 8 of the menstrual cycle. Menstrual records were maintained for all the monkeys during the period of study. Blood samples (3 ml) were collected from either of the two arms or the leg other than the one carrying the implant between 10.00 and 10.30 AM on days 1,3,5,7 and then every fortnight after the implant insertion. Serum was separated and stored at -20% until processed for estimation of NET levels. Radioimmunoassay of NET (17a-ethynyl-178-hydroxy-4-estren-3-one):NET level in serum was estimated by RIA as described previously using the antisera raised against NET-BSA and 15,16-3H-NET (Sp. Act. 57.14 Ci/ mmole) (8,9). The intra-assay and inter-assay coefficient of variation was 9 and 18 per cent, respectively. RESULTS The levels of serum NET and the menstrual bleeding patterns after NETA implant insertion are shown in Fig. l-5. In 3 monkeys (1059,1062 and 1056), serum NET levels showed variation among samples. The NET levels in monkey 1059 (Fig. 1) was above 10 ng until day 130 and thereafter the NET levels were below 10 ng/ml. Monkey No. 1062 (Fig. 2) showed initial rise in NET levels followed by a fall and rise in NET level. The NET level decreased to about 10 ng/ml between day 120 and 175 and during that period, the animal experienced vaginal bleeding episodes at regular intervals comparable to a normal menstrual cycle. This was followed by a second rise in NET level with subsequent fall after implant removal. The serum NET levels of the third monkey (1056) showed an initial rise with fluctuating levels between lo-26 ng/ml until implant was removed (Fig. 3). This monkey had one bleeding episode on day 145 when the NET level was about 10 ng/ml.
328
SEPTEMBER 1991 VOL. 44 NO. 3
CONTRACEPTION
NGNKEY No. lOS!l MENSTRUAL BLEEUNG
JAN
Fig.
MARCH
FEB
APRIL
-
MAY
tiw4owA
JUNE
JULY
AUG.
levels Serum NET (ng/ml) menstrual pattern in monkey no.
1.
and 1059.
30 INSERTION
MONKEY NO. 1062 NNSTRUAL BLCEOlKi
t REMOVAL -
I, 201 P % ti lo-
01
JAN
Fig.
-
1
so
LO
2.
FEE
loo NARCH
120
APRIL
Lo MAY
1601
180
JUNE
1 JULY
220
1 IL0 AUG.
levels Serum NET (ng/ml) menstrual pattern in monkey no.
SEPTEMBER 1991 VOL. 44 NO. 3
OAYS
and 1062.
329
CONTRACEPTION
Monkey Nal056 Menstrual Bleedinq
fRemov0 ,I
I
30
E 3 ;
m z
20
. F I-
U
% 10
0 1I 20
I
I
LO
60
Jan.
Fig.
Feb.
3.
V
1
1
I
100
80 M or.
I
120
Apr.
I
1LO 160 May.
1
160
June
200 July
NET levels Serum @g/ml) menstrual pattern in monkey no.
“1 INSERTION
No.1050 MENSTRUAL BLEEDIM larMEY
I
220 DAYS
and 1056.
~REMWL
I
4 10
2+ JAN
Fig.
330
4.
Lb
sb FEB
“p MARCH
Id0
I
140
APRIL
140 Miv
1c
I
lb0
JUNE
1
220
w
JULY
NET levels Serum (ng/ml) menstrual pattern in monkey no.
and 1050.
SEPTEMBER 1991 VOL. 44 NO. 3
CONTRACEPTION
30 INSERTION
MONKEY
NO. 1053
tREMo\rpL
Menstrualbleeding-
Fig.
5.
levels NET Serum W/ml) menstrual pattern in monkey no.
and 1053.
In the fourth monkey (Fig. 4), the NET level started from 3 ng/ml with spikes of rise until day 110. Subsequently, the NET level went down.below 10 ng/ml and the monkey started cycling. Figure 5 shows the NET levels and menstrual pattern in the 5th monkey. This monkey showed regular bleeding episodes up to 4 months after implant insertion. NET level during the initial 4 months ranged between 2-10 ng/ml serum. The NET level started rising after day 130 and the monkey became amenorrheic. The serum NET level came down around day 203 and the cycle was resumed. DISCUSSION The present study has shown that after the insertion of NETA implants in monkeys , prolonged amenorrhea was observed in 4 out of 5 monkeys. In this study the serum levels of NET in monkeys were significantly higher than the NET levels reported in humans (10,ll). However, the pattern of serum NET levels was similar to NET-releasing subcutaneous rods in human (10). Prasad et al. (12) reported that pharmacokinetic parameters of NET in monkeys were inconsistent and the plasma levels showed fluctuating patterns with time. A number of studies (10,11,13,14) reported that women using NET-releasing devices had high incidence of bleeding. Odlind et al. (10) reported amenorrhea in 2 out of 5 and spotting and bleeding in 3 of the 5 women with NET-releasing subcutaneous rods. In this study with NETA implants, four out of five monkeys were amenorrheic during
SEPTEMBER 1991 VOL. 44 NO. 3
331
CONTRACEPTION the initial 4 months of implant in situ. The altered bleeding patterns in our monkeys could be due to altered ratio of circulating levels of NETA/NET and is quite similar to human subjects. In general, amenorrhea was associated with NET levels greater than 10 ng/ml. Landgren et al. (13) reported that they obtained zero order release with vaginal ring devices based on plasma NET levels. Yet, they observed no correlation between the release rate from the device and plasma levels of NET in the same subject. In the present study, we also found that NET levels varied widely between monkeys as well as within the same monkey at different times. Fotherby et al. (15,16) have reported that pharmacokinetic and pharmacodynamic parameters of two of the injectables, depotmedroxyprogesterone acetate and norethindrone enanthate, have considerable variation in different populations whereas we found that monkeys maintained in the same environmental conditions have different levels of NET due to individual variation. The variation in the serum levels of NET could be due to changes in rate of metabolism, elimination and the amount of sex steroid binding globulin in blood (17). In this study we were not able to measure the levels of serum NETA in monkeys. The observations on the subdermal NETA silastic implants in monkeys and those reported in human showed close similarity in pharmacokinetic and pharmacodynamic parameters and hence reinforce the belief that the rhesus monkey could be used as an ideal pharmacological model for the study of long-acting steroid delivery systems. ACKNOWLEDGEMENT This study was supported by a grant from the Special Programme of Research, Development and Research Training in Human Reproduction of the World Health Organization, Geneva. REFERENCES 1.
Coutinho, E.M., Mattos, C.E.R., Sant'Anna, A.R.S., Adeodata Filho, J., Silva, M.C. and Tatum, H.J. Long-term contraception by subcutaneous silastic capsules containing megestrol acetate. Contraception 2:313-321 (1970).
2.
Bhatnagar, S., Srivastava, U.K., Takkar, D., Chandra, V.L., Hingorani, V. and Laumas, K.R. Long-term contraception by steroid-releasing implants. II. A preliminary report on longterm contraception by a single silastic implant containing norethindrone acetate (ENTA) in women. Contraception 11:505523 (1975).
3.
Moore, D.E., Roy, S., Stanczyk, F.C. and Mishell, D.R. Jr. Bleeding and serum d-norgestrel, oestradiol and progesterone patterns in women using d-norgestrel subdermal polysioloxane capsules for contraception. Contraception 17:315-328 (1978).
332
SEPTEMBER 1991 VOL. 44 NO. 3
CONTRACEPTION
4.
Sivin, I., Sanchez, F.A., Dias, S. et al. Three-year experience with NorplantTM subdermal contraception. Fertil Steril 39:799-808 (1983).
5.
Shahani, S.M., Kulkarni, P.O., Bhati, P.A. and Patel, K.L. Metabolic and endocrine studies in women using norethindrone acetate implant. Contraception 19:135-144 (1979).
6.
Belsey, E.M., Peregoudov, S. and Task Force on Long-acting Systemic Agents for Fertility Regulation. Determinants of menstrual bleeding pattern among women using natural and hormonal methods of contraception. Contraception 38:227242 (1988).
7.
Jha, P., Murugesan, K. and Farooq, A. Particle size and release of norethindrone acetate through silastic tubing : An in vitro and -in vivo study. Int. J. Pharmaceutics 63:143-147 (1990).
a.
Hillier, S.G., Jha, P., Griffiths, K. and Laumas, K.R. Long-term contraception by steroid-releasing implants. VI. Serum concentration of norethindrone in women bearing a single silastic implant releasing norethindrone acetate. Contraception 15:473488 (1977).
9.
Laumas, V., Malik, B.K., Jamal, K. et al. Radioimmunoassay of norethindrone: Serum levels of norethindrone in lactating women after insertion of a single silastic implant releasing norethindrone acetate. Contraception 18:593-605 (1978).
10.
Odlind, V,, Moo-Young, A.J., Gupta, G.N., Weiner, E. and Johansson, Elof D.B. Subdermal norethindrone pellets - A method for contraception. Contraception 19:639-648 (1979).
11.
Landgren, B.M., Balogh, A., Shin, M.W., Lindberg, M. and Diczfalusy, E. Hormonal effects of the 300 ug norethisterone minipill. 2.Daily gonadotropin levels in 43 subjects during a pretreatment cycle and during the second month of NET administration. Contraception 20: 585-606 (1979).
12.
Prasad, K.V.S, Sivakumar, B. and Narasinga Rao, B.S. The rabbit as an animal model to study pharmacokinetics of norethindrone in women. Contraception 20:619-632 (1979).
13.
Landgren, B.M., Oriowo, M.A. and Diczfalusy, E. Pharmacokinetic and pharmacodynamic studies with vaginal devices releasing norethindrone at a constant near zero order. Contraception 24: 29-44 (1981).
14.
Mishell, D.R. Jr., Lumkin, M. and Jackanicz, T. Initial clinical studies of intravaginal rings containing norethindrone and norgestrel. Contraception 12:253-260 (1975).
SEPTEMBER 1991 VOL. 44 NO. 3
333
CONTRACEPTION
15.
Fotherby, K. Variability of pharmacokinetic parameters for contraceptive steroids. J Steroid Biochem 19:817-820 (1983).
16.
Shi, Y-e, He, C-h, Gu, J. and Fotherby, K. Pharmacokinetics of norethisterone in human. Contraception 35:465-476 (1987).
17.
Kuhl, H., Bremser, H.J. and Taubert, H.D. Serum levels and pharmacokinetics of norethisterone after ingestion of lynestrenol: Its relation to dose and stage of the menstrual cycle. Contraception 26:303-316 (1982).
334
SEPTEMBER 1991 VOL. 44 NO. 3
P.Jha, K.Murugesan and A. Farooq Department of Reproductive Biology All India Institute of Medical Sciences New Delhi - 110029, INDIA
ABSTRACT Subdermal silastic implants releasing norethindrone acetate were inserted in five rhesus monkeys for a period of seven months. The serum norethindrone (NET) levels were estimated from blood samples collected fortnightly. There was an initial high level of NET followed by fluctuating levels of NET in 4 out of five monkeys. The serum NET values showed individual variation between time periods as well as between monkeys. The analysis of menstrual cycles showed initial amenorrhoea up to day 120-140 in four out of five monkeys. Only one monkey showed vaginal bleeding pattern comparable to normal menstrual cycles; this monkey had serum NET levels less than 10 ng/ml. The phannacokinetic and pharmacodynamic parameters were similar to those reported in the human female.
INTRODUCTION Long-acting hormonal preparations such as injectables and subdermal silastic devices have been tested in humans (l-4) for fertility regulation and found to be effective from a few months to a few years. As these hormonal contraceptive preparations are long-acting, they are considered as methods of choice for fertility regulation in developing countries where motivation is poor and follow-up is difficult. The calculable microquantity of contraceptive steroids released from the subdermal silastic implants has a number of advantages over the oral or injectable preparations where the woman is exposed to very high doses of contraceptive steroids. Though these subdermal silastic implants were considered to produce lesser side effects, yet menstrual disturbances during the first few cycles were observed in many studies (l-4). The norethindrone acetate (NETA)releasing subdennal implant was tested and found to be effective for a period up to 6 months in humans (5). But, it was shown to have varying menstrual problems. Belsey and associates (6) have studied the menstrual bleeding patterns among contraceptive users and reported varying menstrual patterns with combination oral contraceptives, progestogen only and injectable regimens. The present study was carried out in rhesus monkeys which have menstrual pattern similar to the human female in order to understand the bioavailability of NET after the insertion of NETreleasing subdermal silastic implants and to ascertain whether the Submitted for publication June 18, 1990 Accepted for publication July 17, 1991
SEPTEMBER 1991 VOL. 44 NO. 3
327
CONTRACEPTION circulating NET levels had any correlation with the observed menstrual pattern. The observations made in this study have potential value in developing another useful long-acting contraceptive modality. MATERIALS AND METHODS Implants: The implants were prepared by filling 40 mg NETA in a carefully washed, dried and inspected silastic tubing (Dow-Corning Corporation, Midland, Michigan) of 22 mm length and 0.59 mm wall thickness (7). Animals: Five adult healthy female rhesus monkeys weighing between 4.5-5.5 kg, maintained at the Primate Research Facility of AIIMS were used for this study. Monkeys with at least two consecutive menstrual cycles of normal duration were selected for the study. They were kept in a single room and were given the same diet. Monkeys were inserted subdennally with silastic implants releasing NETA on the lateral side of the thigh under general anaesthesia between day 4 to 8 of the menstrual cycle. Menstrual records were maintained for all the monkeys during the period of study. Blood samples (3 ml) were collected from either of the two arms or the leg other than the one carrying the implant between 10.00 and 10.30 AM on days 1,3,5,7 and then every fortnight after the implant insertion. Serum was separated and stored at -20% until processed for estimation of NET levels. Radioimmunoassay of NET (17a-ethynyl-178-hydroxy-4-estren-3-one):NET level in serum was estimated by RIA as described previously using the antisera raised against NET-BSA and 15,16-3H-NET (Sp. Act. 57.14 Ci/ mmole) (8,9). The intra-assay and inter-assay coefficient of variation was 9 and 18 per cent, respectively. RESULTS The levels of serum NET and the menstrual bleeding patterns after NETA implant insertion are shown in Fig. l-5. In 3 monkeys (1059,1062 and 1056), serum NET levels showed variation among samples. The NET levels in monkey 1059 (Fig. 1) was above 10 ng until day 130 and thereafter the NET levels were below 10 ng/ml. Monkey No. 1062 (Fig. 2) showed initial rise in NET levels followed by a fall and rise in NET level. The NET level decreased to about 10 ng/ml between day 120 and 175 and during that period, the animal experienced vaginal bleeding episodes at regular intervals comparable to a normal menstrual cycle. This was followed by a second rise in NET level with subsequent fall after implant removal. The serum NET levels of the third monkey (1056) showed an initial rise with fluctuating levels between lo-26 ng/ml until implant was removed (Fig. 3). This monkey had one bleeding episode on day 145 when the NET level was about 10 ng/ml.
328
SEPTEMBER 1991 VOL. 44 NO. 3
CONTRACEPTION
NGNKEY No. lOS!l MENSTRUAL BLEEUNG
JAN
Fig.
MARCH
FEB
APRIL
-
MAY
tiw4owA
JUNE
JULY
AUG.
levels Serum NET (ng/ml) menstrual pattern in monkey no.
1.
and 1059.
30 INSERTION
MONKEY NO. 1062 NNSTRUAL BLCEOlKi
t REMOVAL -
I, 201 P % ti lo-
01
JAN
Fig.
-
1
so
LO
2.
FEE
loo NARCH
120
APRIL
Lo MAY
1601
180
JUNE
1 JULY
220
1 IL0 AUG.
levels Serum NET (ng/ml) menstrual pattern in monkey no.
SEPTEMBER 1991 VOL. 44 NO. 3
OAYS
and 1062.
329
CONTRACEPTION
Monkey Nal056 Menstrual Bleedinq
fRemov0 ,I
I
30
E 3 ;
m z
20
. F I-
U
% 10
0 1I 20
I
I
LO
60
Jan.
Fig.
Feb.
3.
V
1
1
I
100
80 M or.
I
120
Apr.
I
1LO 160 May.
1
160
June
200 July
NET levels Serum @g/ml) menstrual pattern in monkey no.
“1 INSERTION
No.1050 MENSTRUAL BLEEDIM larMEY
I
220 DAYS
and 1056.
~REMWL
I
4 10
2+ JAN
Fig.
330
4.
Lb
sb FEB
“p MARCH
Id0
I
140
APRIL
140 Miv
1c
I
lb0
JUNE
1
220
w
JULY
NET levels Serum (ng/ml) menstrual pattern in monkey no.
and 1050.
SEPTEMBER 1991 VOL. 44 NO. 3
CONTRACEPTION
30 INSERTION
MONKEY
NO. 1053
tREMo\rpL
Menstrualbleeding-
Fig.
5.
levels NET Serum W/ml) menstrual pattern in monkey no.
and 1053.
In the fourth monkey (Fig. 4), the NET level started from 3 ng/ml with spikes of rise until day 110. Subsequently, the NET level went down.below 10 ng/ml and the monkey started cycling. Figure 5 shows the NET levels and menstrual pattern in the 5th monkey. This monkey showed regular bleeding episodes up to 4 months after implant insertion. NET level during the initial 4 months ranged between 2-10 ng/ml serum. The NET level started rising after day 130 and the monkey became amenorrheic. The serum NET level came down around day 203 and the cycle was resumed. DISCUSSION The present study has shown that after the insertion of NETA implants in monkeys , prolonged amenorrhea was observed in 4 out of 5 monkeys. In this study the serum levels of NET in monkeys were significantly higher than the NET levels reported in humans (10,ll). However, the pattern of serum NET levels was similar to NET-releasing subcutaneous rods in human (10). Prasad et al. (12) reported that pharmacokinetic parameters of NET in monkeys were inconsistent and the plasma levels showed fluctuating patterns with time. A number of studies (10,11,13,14) reported that women using NET-releasing devices had high incidence of bleeding. Odlind et al. (10) reported amenorrhea in 2 out of 5 and spotting and bleeding in 3 of the 5 women with NET-releasing subcutaneous rods. In this study with NETA implants, four out of five monkeys were amenorrheic during
SEPTEMBER 1991 VOL. 44 NO. 3
331
CONTRACEPTION the initial 4 months of implant in situ. The altered bleeding patterns in our monkeys could be due to altered ratio of circulating levels of NETA/NET and is quite similar to human subjects. In general, amenorrhea was associated with NET levels greater than 10 ng/ml. Landgren et al. (13) reported that they obtained zero order release with vaginal ring devices based on plasma NET levels. Yet, they observed no correlation between the release rate from the device and plasma levels of NET in the same subject. In the present study, we also found that NET levels varied widely between monkeys as well as within the same monkey at different times. Fotherby et al. (15,16) have reported that pharmacokinetic and pharmacodynamic parameters of two of the injectables, depotmedroxyprogesterone acetate and norethindrone enanthate, have considerable variation in different populations whereas we found that monkeys maintained in the same environmental conditions have different levels of NET due to individual variation. The variation in the serum levels of NET could be due to changes in rate of metabolism, elimination and the amount of sex steroid binding globulin in blood (17). In this study we were not able to measure the levels of serum NETA in monkeys. The observations on the subdermal NETA silastic implants in monkeys and those reported in human showed close similarity in pharmacokinetic and pharmacodynamic parameters and hence reinforce the belief that the rhesus monkey could be used as an ideal pharmacological model for the study of long-acting steroid delivery systems. ACKNOWLEDGEMENT This study was supported by a grant from the Special Programme of Research, Development and Research Training in Human Reproduction of the World Health Organization, Geneva. REFERENCES 1.
Coutinho, E.M., Mattos, C.E.R., Sant'Anna, A.R.S., Adeodata Filho, J., Silva, M.C. and Tatum, H.J. Long-term contraception by subcutaneous silastic capsules containing megestrol acetate. Contraception 2:313-321 (1970).
2.
Bhatnagar, S., Srivastava, U.K., Takkar, D., Chandra, V.L., Hingorani, V. and Laumas, K.R. Long-term contraception by steroid-releasing implants. II. A preliminary report on longterm contraception by a single silastic implant containing norethindrone acetate (ENTA) in women. Contraception 11:505523 (1975).
3.
Moore, D.E., Roy, S., Stanczyk, F.C. and Mishell, D.R. Jr. Bleeding and serum d-norgestrel, oestradiol and progesterone patterns in women using d-norgestrel subdermal polysioloxane capsules for contraception. Contraception 17:315-328 (1978).
332
SEPTEMBER 1991 VOL. 44 NO. 3
CONTRACEPTION
4.
Sivin, I., Sanchez, F.A., Dias, S. et al. Three-year experience with NorplantTM subdermal contraception. Fertil Steril 39:799-808 (1983).
5.
Shahani, S.M., Kulkarni, P.O., Bhati, P.A. and Patel, K.L. Metabolic and endocrine studies in women using norethindrone acetate implant. Contraception 19:135-144 (1979).
6.
Belsey, E.M., Peregoudov, S. and Task Force on Long-acting Systemic Agents for Fertility Regulation. Determinants of menstrual bleeding pattern among women using natural and hormonal methods of contraception. Contraception 38:227242 (1988).
7.
Jha, P., Murugesan, K. and Farooq, A. Particle size and release of norethindrone acetate through silastic tubing : An in vitro and -in vivo study. Int. J. Pharmaceutics 63:143-147 (1990).
a.
Hillier, S.G., Jha, P., Griffiths, K. and Laumas, K.R. Long-term contraception by steroid-releasing implants. VI. Serum concentration of norethindrone in women bearing a single silastic implant releasing norethindrone acetate. Contraception 15:473488 (1977).
9.
Laumas, V., Malik, B.K., Jamal, K. et al. Radioimmunoassay of norethindrone: Serum levels of norethindrone in lactating women after insertion of a single silastic implant releasing norethindrone acetate. Contraception 18:593-605 (1978).
10.
Odlind, V,, Moo-Young, A.J., Gupta, G.N., Weiner, E. and Johansson, Elof D.B. Subdermal norethindrone pellets - A method for contraception. Contraception 19:639-648 (1979).
11.
Landgren, B.M., Balogh, A., Shin, M.W., Lindberg, M. and Diczfalusy, E. Hormonal effects of the 300 ug norethisterone minipill. 2.Daily gonadotropin levels in 43 subjects during a pretreatment cycle and during the second month of NET administration. Contraception 20: 585-606 (1979).
12.
Prasad, K.V.S, Sivakumar, B. and Narasinga Rao, B.S. The rabbit as an animal model to study pharmacokinetics of norethindrone in women. Contraception 20:619-632 (1979).
13.
Landgren, B.M., Oriowo, M.A. and Diczfalusy, E. Pharmacokinetic and pharmacodynamic studies with vaginal devices releasing norethindrone at a constant near zero order. Contraception 24: 29-44 (1981).
14.
Mishell, D.R. Jr., Lumkin, M. and Jackanicz, T. Initial clinical studies of intravaginal rings containing norethindrone and norgestrel. Contraception 12:253-260 (1975).
SEPTEMBER 1991 VOL. 44 NO. 3
333
CONTRACEPTION
15.
Fotherby, K. Variability of pharmacokinetic parameters for contraceptive steroids. J Steroid Biochem 19:817-820 (1983).
16.
Shi, Y-e, He, C-h, Gu, J. and Fotherby, K. Pharmacokinetics of norethisterone in human. Contraception 35:465-476 (1987).
17.
Kuhl, H., Bremser, H.J. and Taubert, H.D. Serum levels and pharmacokinetics of norethisterone after ingestion of lynestrenol: Its relation to dose and stage of the menstrual cycle. Contraception 26:303-316 (1982).
334
SEPTEMBER 1991 VOL. 44 NO. 3