A Comparison of the Pharmacokinetic Properties of Streptokinase and Anistreplase in Acute Myocardial Infarction

Br. J. clin. Pharmac. (1991), 31, 143-147 AD 0NI S 030652519100028R

A comparison of the pharmacokinetic properties of streptokinase and anistreplase in acute myocardial infarction

J. D. GEMMILL', K. J. HOGG', J. M. A. BURNS', A. P. RAE', F. G. DUNN', R. FEARS2, H. FERRES2, R. STANDRING2, H. GREENWOOD2, D. PIERCE3 & W. S. HILLIS' 'Departments of Medicine and Therapeutics and Cardiology, University of Glasgow, Stobhill General Hospital, Balornock Road, Glasgow G21 3UW, 2Beecham Pharmaceuticals Research Division, Great Burgh, Epsom, Surrey KT18 5XQ and 3Beecham Pharmaceuticals Research Division, Coldharbour Road, The Pinnacles, Harlow, Essex CM19 SAD

1 The pharmacokinetics of streptokinase (SK) and anistreplase in conventional dosage regimens of 1.5 x 106 i.u. of SK infused over 60 min and 30 units of anistreplase over 5 min were studied in 24 consecutive patients presenting with acute myocardial infarction, using a functional bioassay to assess concentrations. 2 The two agents were found to have similar volumes of distribution (5.68 and 5.90 1), but SK was cleared significantly more rapidly than anistreplase, resulting in a shorter terminal phase half-life (0.61 vs 1.16 h) and a shorter mean residence time (0.76 vs 1.55 h).

Keywords streptokinase myocardial infarction pharmacokinetics anistreplase

Introduction The role of thrombolytic therapy in the treatment of profile variously fitted by mono- or bi-exponential func- acute myocardial infarction is well established, and the tions (Kohler et al., 1987, Mentzer et al., 1986), while earlier treatment is initiated, the greater the clinical anistreplase is eliminated from the circulation much benefits (GISSI, 1986; ISAM, 1986; ISIS II, 1987; AIMS, more slowly than SK (Been et al., 1986; Kohler et al., 1988). 1987). In this study, we have compared the pharmaco- The ideal thrombolytic agent would therefore be easily kinetic properties of these agents directly using a func- and rapidly administered by the intravenous route and tional bioassay. have pharmacokinetic properties allowing rapid onset of thrombolytic activity maintained for long enough to prevent early coronary reocclusion. Currently avail- able agents and dosage regimens are not ideal, but Methods pharmacokinetic studies may facilitate the development of optimal administration regimens. Twenty-four consecutive patients (18 male, six female, Streptokinase binds to plasminogen (or plasmin) in age range 48 to 72 years) with acute myocardial infarc- the blood to form the complex SK-glu-plasminogen tion as judged by strict ECG criteria, presenting within (-plasmin) which is an effective plasminogen activator 6 h of onset of pain, and without contraindications to (Anderson et al., 1987). Anisoylated lys-plasminogen thrombolytic therapy were treated with either a conven- streptokinase activator complex (APSAC, anistreplase, tional dose of 1.5 x 106 i.u. of SK infused intravenously Eminase, a trademark of Beecham Group p.l.c.) is a over 60 min or 30 u of anistreplase as a 5 min continuous pro-enzyme giving rise to the plasminogen activator intravenous injection. Blood samples were obtained complex streptokinase-lys-plasminogen by deacylation. from an indwelling venous catheter at frequent intervals The activator complexes of the two agents are of com- up to 24 h after dosing (0,6,10,20,30,45,60,75,90 min, parable potency in the activation of plasminogen. 2, 4, 6, 9, 12 and 24 h). They were collected into 0.1 Deacylation of anistreplase occurs with a half-life of volumes of 3.8% w/v sodium citrate, the plasma was 105 min in vitro and is thought to be rate-limiting for the separated immediately at 40 C and stored at -70° C. removal of anistreplase from the circulation, and to be Total fibrinolytic activity was measured as described slower than the elimination of SK-plasminogen or SK. by Been et al. (1986) and Nunn et al. (1987), and used as Previous pharmacokinetic studies are relatively few in a functional bioassay of the plasma concentrations of the number, were flawed by the paucity and timing of blood thrombolytic agents. The preparation of euglobulin samples, and have used a variety of different assay fractions has been reported in detail elsewhere (Standring methods. Despite these limitations, they have suggested et al., 1988). In brief, plasma samples were diluted with that SK is rapidly eliminated with a concentration-time 0.011% v/v acetic acid and the resulting precipitates 143 144 J. D. Gemmill et al.

Table 1 Pharmacokinetic parameters Anistreplase (n = 12) SK (n = 12) Mean (s.d.) Mean (s.d.) tmax h 0.15 (0.07) 0.9 (0.21)** Cmax (M X 10 8) 5.59 (2.22) 3.85 (1.18)t Volume of distribution (1) Model-independent (V) 5.90 (1.91) 5.68 (2.29)NS Computer-modelled (Vss) 5.25 (1.49) Clearance (1 h-I) Model-independent (CL) 3.87 (1.52) 7.08 (2.91)* Computer-modelled (CL) 3.72 (1.35) Terminal phase elimination half-life (h) Model-independent t½2 1.16 (0.38) 0.61 (0.24)** Computer-modelled t½12 1.15 (0.38) Time to half maximalfibrinolytic activity (h) 1.12 (0.31) 0.48 (0.14)** Mean residence time (h) 1.55 (0.48) 0.76 (0.31)** tP < 0.05, *P < 0.01, **P < 0.001, NS = Not significant. were solubilized to give a 30-fold dilution of the original Powell algorithm (Allen, 1990). The same data sets were plasma. This dilution factor was found to abolish the subjected to compartmental model-independent analysis interference from variable amounts of endogenous calculating AUC using the linear trapezoidal method plasminogen and plasmin in the samples (Nunn et al., with extrapolation to infinity. Clearance (CL) and 1987). Fibrinolytic activity was assayed by the lysis of volume of distribution (V) were calculated by standard fibrin plates prepared from human fibrinogen (containing methods (Gibaldi & Perrier, 1982). The terminal phase 2 ,ug plasminogen mg-' fibrinogen) incubated at 370 C half-life (t½l,) was also determined by regression analysis for at least 18 h. The plates were stained with bromo- as was the time to loss of half the maximal fibrinolytic phenol blue and lysis zones were measured with an AMS activity from the end ofthe dosing period. Instantaneous image analyser. Quadruplicate measurements were mean residence times (MRT), which compensate for the made at each time point and typical coefficients of differences in the duration of dosing between SK and variation ranged from 0.44 to 2.6% (mean 1.6%, n = 11) anistreplase, were determined by moment analysis for anistreplase and from 0.36 to 6.6% (mean 2.4%, (Riegelman & Collier, 1980). n = 10) for SK. The concentration of activator was Pharmacokinetic parameters for anistreplase and SK calculated for each patient employing the appropriate (V, t½/2 and CL) were compared by one-way analysis of standards diluted with autologous predose plasma, i.e. variance. SK for SK treated patients and anistreplase for anistreplase treated patients. Standards were prepared using the patient's own pretreatment plasma to allow for any interpatient variability; the slopes of the standards Results ranged from 1.57 to 3.26 (mean 2.42) for SK, and 1.78 to 3.07 (mean 2.47) for anistreplase. SK and anistreplase Streptokinase standards gave linear responses with correlation coefficients of 0.9946 (percentage coefficient of variation, Derived pharmacokinetic parameters are summarised in %CV 0.34) for SK (n = 12) and 0.9938 (%CV 0.21) for Table 1, and mean concentration-time curves are shown anistreplase (n = 12). in Figure 1. Concentration-time curves during and after Internal standards were also included in each assay, in infusion were analysed by model-independent methods pretreatment, and 10 min to 2 h post-treatment samples, only as the data could not be adequately fitted by a and processed concurrently. The average recovery for compartmental model. The terminal phase rate constant samples from the entire group of 12 patients was 91% for was determined using non-linear regression analysis SK and 94% for anistreplase. The limit of reliable assay with a weighting of concentration-2. was 0.08 x 10- M (3.91 i.u. ml-') for 10 SK treated The maximum plasma concentration (Cmax) occurred patients and 0.04 x 10-8 M (1.95 i.u. ml-') for the within 1.25 h (tmax) ofthe start ofthe infusion (mean 0.9 h). remaining two. For anistreplase the lower limit of assay Subsequently concentrations declined rapidly to less was 0.06 x 10- M (7.81 x 10-5 i.u. ml-1). than 15% of Cmax in all subjects by 4 h after the start of Plasma SK- and anistreplase-time data were fitted by infusion, and in most subjects were below the limit of one- or two-compartment models using non-linear re- reliable measurement at this time. The post peak decline gression as appropriate to each individual data set. in concentration approximated to a mono-exponential Modelling was performed using the computer program, fall (maximum % CV of the regression lines was 26%) as MODFIT, which employs a modified Danielson-Fletcher- determined by MODFIT (Allen, 1989). Pharmacokinetics ofstreptokinase and anistreplase 145

U) 80 _- 80 r 0 CO) Q x a)

U) i 60 0. 60 en 0 C CY) C .° 40 40 Cn x Cu

C 0 2 a) c 20 U1) 20k E Un cL O 0L _RK infiiqinn II 0 60 120 180 240 300 360 0 60 120 180 240 300 360 Time (min) Time (min)

Figure 1 Plasma concentrations of streptokinase (mean + Figure 2 Plasma concentrations of anistreplase (mean ± s.d.) s.d.) during and after intravenous infusion of 1.5 x 106 i.u. of following intravenous injection of 30 u of anistreplase over streptokinase over 60 min. 5 min.

The volume of distribution of SK was low, approxi- 1985; Khalilullah et al., 1984; Lew et al., 1986; Weinstein, mating to that of plasma proteins. In conjunction with 1982), and is associated with greatest clinical benefits a moderately rapid clearance this accounted for a short (AIMS, 1988; Anderson et al., 1984; GISSI, 1986; terminal phase half-life. ISAM, 1986; ISIS II; 1987, Simoons et al., 1985). For earlier institution of therapy, a suitable thrombolytic Anistreplase agent must be administered in a simple regimen allowing easy administration, applicable even outside the hospital The maximum total activity of anistreplase (Cmax) environment. The ideal regimen should therefore achieve occurred within 20 min (tmax) of the start of the 5 min a high early concentration, and resulting fibrinolytic intravenous injection (mean 0.15 h). activity (short tmax and high Cmax), a slow elimination Activity fell to less than 15% of maximum by 4 h. In phase, allowing maintenance of adequate concentrations six of the 12 patients, an early rapid decline phase of the from a single 'bolus' injection (long t½l, low CL) for an concentration-time curve could be delineated, conform- appropriate period of time, thus minimising early re- ing to a two-compartment model. In five subjects the occlusion. data sets conformed to a one-compartment model. In Early recommendations for the use of SK were for a one subject, neither model provided a satisfactory fit prolonged low dose infusion, but more recent usage in and these data were analysed by model-independent myocardial infarction has adopted the now standard, but methods only. The derived parameters were similar essentially empirical dose of 1.5 x 106 i.u. over 60 min. regardless of which model was most appropriate, and The recommended dose of anistreplase was based on were comparable with parameters derived from model- being equivalent to 1-1.5 x 106 i.u. of SK. Following the independent analysis. administration of SK and anistreplase in their standard Pharmacokinetic parameters are summarised in Table treatment regimens for myocardial infarction, anistreplase 1 and mean total activity-time curves are shown in achieves a significantly earlier and higher peak concen- Figure 2. tration as would be predicted from its more rapid infusion. The volume of distribution of anistreplase and its In this study, we have found that SK and anistreplase deacylated product was relatively low, consistent with both have a low volume of distribution, approximately anistreplase and its metabolite being confined largely to twice that of plasma volume and similar to the volume the systemic circulation. Clearance was modest and in of distribution of plasma proteins (Rowland & Tozer, conjunction with a small volume of distribution resulted 1980). This would be consistent with both agents in a longer elimination phase half-life (1.16 ± 0.38 h) behaving as proteins with no specific carrier mechanism. than that of SK. The observed elimination phase half-life of SK of 0.61 h was similar to previous estimates based on fibrin plate lysis assay in acute myocardial infarction (Kohler et al., 1987) and slightly longer than estimates based on Discussion other functional assays of0.3 h (Martin, 1982) and 0.38 h (Mentzer et al., 1986). These small differences may be Thrombolytic therapy in acute myocardial infarction accounted for by variation in assay specificity and phar- achieves infarct related artery patency, limitation of macokinetic analysis. Claims of a considerably longer infarct size and improves mortality. Coronary reperfusion terminal phase, with a half-life of 1.38 h (Fletcher et al., rates are highest when the therapy is administered early 1958; Grierson & Bjornson, 1987), are based on radio- in the course of myocardial infarction (Kennedy et al., immunoassay or amidolytic assay methods for SK. 146 J. D. Gemmill et al.

These methods do not differentiate between active and measures the functional moiety and is relevant to the inactive SK fragments, or take account of in vivo de- clinical application of these drugs. iodination. Similarly, the amidolytic assays based on We have shown that SK and anistreplase have similar the lysis of chromogenic substrate do not evaluate the low volumes of distribution, but that SK is effectively fibrinolytic sites of the activator molecule, and also cleared from the circulation twice as fast as anistreplase therefore cannot differentiate active and inactive SK (7.08 vs 3.87 1 h-1, P < 0.01). In vitro studies of the fragments, or fragments bound by circulating inhibitors deacylation of anistreplase in human blood or plasma such as a2-antiplasmin. The results of previous studies have shown a deacylation half-life of 1.76 h (Ferres, based on the lysis of bovine fibrin are also problematic, 1987). Our results would therefore support the concept in that this assay is particularly sensitive to endogenous that the deacylation of anistreplase is the rate-limiting human plasminogen. step in its elimination. In this study, both drugs have been assayed by the Both streptokinase as a 60 min infusion of 1.5 x 106 same fibrinolytic bioassay based on human fibrin plate i.u. (Verstraete et al., 1985) and anistreplase as a 5 min lysis. It has been shown that administration of either injection of 30 units (Been etal., 1985; Hillis et al., 1987; thrombolytic agent does not completely deplete circu- Ikram etal., 1986; Timmis etal., 1987) are well-tolerated lating plasminogen. Therefore, all of the SK present and effective in restoring coronary patency in acute would be in its fibrinolytically active complexed form. myocardial infarction. In this study, we have confirmed Thirty-fold dilution of the euglobulin fractions eliminates that anistreplase in this dosage schedule achieves earlier, the influence of endogenous plasminogen on the assay higher and more prolonged levels of total thrombolytic procedure (Fears, 1989). Therefore, this assay method activity.

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