INTRODUCTION

Top Abstract Introduction Materials and Methods Results Discussion References

Respiratory syncytial virus (RSV) remains one of the most important infectious causes of hospitalization of infants and young children (8). Seasonal outbreaks of RSV disease result in infection in over 90% of children by 2 years of age. The outcome varies from a mild upper respiratory tract illness in approximately 75% of infected infants and children to a severe, life-threatening disease in a small percentage of infected patients. Infants at particular risk of severe disease include prematurely born children, children born with congenital heart disease, and children with chronic lung disease, particularly bronchopulmonary dysplasia (BPD) (10).

Despite the importance of RSV as a pathogen in the pediatric age group, options for treatment and prevention of RSV disease are limited. Active immunization to prevent severe RSV disease has not yet proven successful (7). Two clinical trials with a polyclonal, hyperimmune RSV globulin have demonstrated safety and efficacy for prophylaxis in carefully selected children at increased risk for severe RSV infection (6, 13). The results of these clinical trials led to Food and Drug Administration licensure of this RSV antibody-enriched immune globulin in January 1996.

A second-generation product for immunoprophylaxis against RSV disease is a monoclonal antibody directed against a highly immunogenic RSV surface glycoprotein (12, 15). Monoclonal antibodies to the fusion protein have been shown to be effective in preventing RSV infection in animal models (3, 17-19) and in humans (9). During the 1995-1996 respiratory virus season, a multicenter, double-blind, placebo-controlled, randomized trial was conducted with SB 209763, a humanized murine monoclonal antibody directed against the RSV fusion glycoprotein. The primary objectives of this phase I trial were to evaluate the safety and pharmacokinetics of single and repeat intramuscular (i.m.) doses of SB 209763 in a pediatric population at risk for severe RSV disease. The secondary objectives included measurement of fusion inhibition titers and neutralization titers and assessment of anti-idiotypic anti-SB 209763 antibodies.

(This study was presented in part at the 37th Interscience Conference on Antimicrobial Agents and Chemotherapy, Toronto, Ontario, Canada, 28 September to 1 October 1997.)

	MATERIALS AND METHODS

Top Abstract Introduction Materials and Methods Results Discussion References

Patients. Infants and young children less than 37 months of age were eligible for enrollment at one of the seven participating sites if they were born prematurely (35 weeks), with a chronological age of 6 months, and/or had a history of BPD. Eligible subjects were required to have a life expectancy of at least 6 months. The protocol was approved by the institutional review board at each site, and informed consent was obtained from all participating families. Exclusion criteria included known preexisting heart, liver, or renal disease; a recognized immune system abnormality; severe respiratory illness requiring assisted ventilation; or previous gamma globulin infusion.

Randomization. Participants were randomly assigned to one of four monoclonal antibody dosage groups (0.25, 1.25, 5.0, or 10.0 mg of SB 209763 per kg of body weight) or to a placebo group according to a randomization schedule provided by SmithKline Beecham. Each dosage group included at least eight subjects; within each dosage group, at least six infants received SB 209763 at the first administration while at least two subjects received the placebo (Table 1). Eight weeks after administration of the first dose, a second i.m. injection was given. Subjects who were randomized to received the monoclonal antibody initially received a repeat dose identical to the initial dose. Subjects who initially received the placebo were crossed over and received SB 209763 at a dosage consistent with that for the group to which they were initially assigned.

Study drug. SB 209763 lyophilate (lot no. U-94045), reconstituted with sterile water to a concentration of 45 mg/ml, and a placebo (a solution containing all components of the active preparation except SB 209763) were used in this study. Doses of 0.25 and 1.25 mg of SB 209763/kg were administered as single i.m. injections into one thigh muscle. Doses of 5.0 and 10.0 mg/kg were divided and administered as separate injections into each thigh muscle to maintain reasonable injection volumes. Placebo subjects were injected with a volume of placebo equivalent to the volume of active drug that would have been received by a subject of identical weight assigned to that dosage group. At the highest dose, a volume of 0.22 ml of either active drug or placebo per kg was administered.

Safety parameters. Screening visits included a medical history and physical examination. A physical examination was performed and blood and urine samples were collected for laboratory tests for determination of patient safety prior to administration of each dose of the study medication. Blood and urine samples were also obtained for laboratory testing at 24 h and 2 weeks after administration of the first dose and at 2 weeks after administration of the second dose of the study medication. A physical examination was also performed at the 2-week-postdose visit. Injection sites were examined for local reactions at approximately 5 min, 2 h, and between 4 and 7 days after each injection. Vital signs were measured prior to and at 15, 30, 45, 60 and 120 min following administration of each dose of the study drug.

6

Pharmacokinetic parameters. For analysis of plasma SB 209763 concentrations, venous blood samples were collected in sodium citrate tubes prior to administration of the first dose and at approximately 24 h, 6 days, and 2 and 8 weeks after the first dose of study medication was administered. The 8-week sample was drawn just prior to administration of the second dose. Blood samples were also drawn at approximately 6 days and 2 and 8 weeks after administration of the second dose of the study medication.

Pharmacodynamic parameters. Venous blood samples for analysis of fusion inhibition and plasma neutralization antibody titers were obtained prior to administration of each dose and at approximately 6 days and 2 and 8 weeks after administration of SB 209763 or the placebo. In vitro microneutralization assays were used to measure the ability of SB 209763 to inhibit the growth of RSV (4). Fusion inhibition and plasma neutralization titers were expressed as the reciprocal of the dilution which caused a 50% reduction in the optical density at 450 nm based on regression analysis of the dose titration.

	RESULTS

Top Abstract Introduction Materials and Methods Results Discussion References

Enrollment. Forty-three infants and children were enrolled in this study (11 premature infants without BPD and 32 infants with BPD). The subjects ranged in age from 8 days to 33 months and weighted between 1.1 and 15.5 kg (mean, 5.3 kg) at the time of enrollment. The racial demographics were as follows: 74% white, 12% black, 12% Oriental, and 2% other. Forty-two subjects completed the study. One subject was withdrawn due to noncompliance with the follow-up protocol after receiving one 10-mg/kg dose of SB 209763. This subject was included in the analysis as if the 10-mg/kg treatment regimen had been completed. The number of subjects who received each treatment regimen is shown in Table 1. The patient demographics as a function of dosage group are shown in Table 2. Although it is difficult to assess homogeneity across dosage groups given the relatively small sample size, it appears that the groups were fairly well balanced in terms of mean age and weight. The racial compositions of the groups were also relatively balanced; participants in all four dosage groups were predominantly white. The gender distributions of the three lowest-dosage groups were similar, with more females being enrolled than males; the highest-dosage group was skewed in the opposite direction. The majority of subjects in each group had chronic lung disease (BPD).

Safety results. Single and repeat doses of 0.25 to 10.0 mg of SB 209763 per kg were safe and well tolerated. There were no deaths during the study. There were 229 adverse events reported for 41 subjects over the 16 weeks following treatment with the study medication. Thirty-seven adverse events occurred in 10 subjects who received the placebo, and 192 were reported in 35 subjects who received SB 209763. Most were mild to moderate in severity. Four adverse events were considered by the investigators to be related to the study drug. These included three episodes of mild to moderate purpura, which occurred in two subjects, and thrombocytosis, which occurred in one subject. The purpura developed at the site of blood sampling. All adverse events had resolved by the end of the study surveillance period. Nineteen laboratory test values were considered clinically relevant by the investigator. Two of these, granulocytopenia and thrombocytosis, were considered to be related to the study medication. The administration of the study drug was not altered in any patient due to side effects.

RSV infection. Sixty-five episodes of respiratory illness occurred. During 40 of these episodes, nasopharyngeal specimens were obtained, and RSV was recovered from nine specimens, as shown in Table 3. Although there were fewer RSV infections in the group receiving SB 209763 at 10 mg/kg (1 of 22) than in the placebo group (2 of 10), the difference was not statistically significant (P = 0.20).

Pharmacokinetic evaluation. Following noncompartmental analysis of the SB 209763 concentration data, C_max values and total exposure (as indicated by the AUC) of SB 209763 following i.m. administration increased with increasing dosages of SB 209763 (Table 4). All subjects had quantifiable concentrations of SB 209763 at the last sampling times for the first and second doses of SB 209763. The T_1/2 was calculated only if at least three time-concentration values were present in the terminal phase. This resulted in the T_1/2 being estimated for 21 of 75 concentration-time profiles derived from 43 subjects, ranging from 22 to 50 days. Concentrations of SB 209763 were available on days 1 and 6 after the first dose of SB 209763 was administered in 19 of the 43 subjects. All 19 of the subjects had higher concentrations on day 6 than on day 1. Mean concentrations in specimens obtained on day 56 (± 7 days), as shown in Table 4, ranged from 0.50 to 21.1 µg/ml for dosages of 0.25 to 10 mg/kg. Mean plasma SB 209763 concentrations following single and repeat i.m. doses are shown in Fig. 1.

View larger version (19K): [in this window] [in a new window]   FIG. 1.   Mean SB 209763 concentrations, ± standard deviations, versus time for subjects receiving doses of 0.25 mg/kg (), 1.25 mg/kg (), 5 mg/kg (), or 10 mg/kg ().

0.316

0.306

Pharmacodynamic evaluation. The mean and median neutralization and fusion inhibition antibody titers on day 6 are shown, in comparison to the corresponding predose titers, in Table 5. Increases in most of the titers were observed for recipients of each of the four dosing regimens. In the 10 placebo recipients, median titers did not significantly change between predose and 6-day-postdose specimens. Among infants who received a dose of 10 mg/kg, there was typically a two- to fourfold increase over predose titers. The wide range of predose neutralization and fusion inhibition antibody titers among infants in different dosing groups explains at least some of the variation in peak titers after dosing.

	DISCUSSION

Top Abstract Introduction Materials and Methods Results Discussion References

In the absence of a safe and effective vaccine, passive immunoprophylaxis holds the greatest promise for the prevention of severe RSV illness in high-risk children. Studies have shown that immunoprophylaxis with a polyclonal, hyperimmune RSV globulin is effective in attenuating RSV disease both in animal models and in high-risk infants and children. However, there are several disadvantages to this product, including the need for intravenous administration, the fluid volume associated with its administration, the less-than-complete protection against RSV disease achieved, and the issue of safety in children with cyanotic heart disease (11). An effective monoclonal antibody can be administered i.m., thereby avoiding many of these difficulties.

Monoclonal antibodies of purely murine origin have not been widely used in clinical trials due to concerns with regard to the development of anti-murine antibodies. Substitution of human sequences for the murine sequences within the non-antigen-binding region of the molecule results in a lower degree of immunogenicity relative to that of unmodified murine monoclonal antibodies. In the present trial, single and repeat doses of 0.25 to 10.0 mg of SB 209763, a humanized murine monoclonal antibody against the RSV fusion protein, per kg were safe and well tolerated when administered i.m. to high-risk infants. Evaluation of the local and systemic anti-SB 209763 immune response failed to reveal evidence of an immune reaction against this humanized antibody. These results are similar to those of previous studies of healthy male subjects who received single and multiple doses of SB 209763 and did not develop a detectable anti-idiotypic reaction (4). Another humanized RSV monoclonal antibody, directed against the F protein, showed a similar lack of immunogenicity (16).

Although the number of sampling times in this study was limited (n = 7) in comparison to many studies examining pharmacokinetics, sufficient information was available to provide individual values of C_max and AUC_0-t by utilizing noncompartmental pharmacokinetic methods. However, parameter estimates for AUC_0-t and C_max should be considered as approximations of the values that would be obtained with a more intensive sampling schedule. By the use of population modeling, estimates of CL/F and V_SS/F were obtainable. The predicted CL/F of 0.143 (ml/h)/kg for the mean age of 298 days is similar to values of intravenous clearance (CL) reported for healthy adults who were administered intravenous doses of SB 209763 (15). In this earlier study, mean CL values ranged from 0.122 to 0.142 (ml/h)/kg for similar doses of 0.25 to 10.0 mg/kg. Mean estimates for V_SS/F from the earlier, intravenous study of healthy adults ranged from 87.5 to 104 ml/kg for doses of 0.25 to 10.0 mg/kg. This is in contrast to a mean V_ss/F of 161 ml/kg for the present study population. These differences could be in part due to the larger proportion of total and extracellular body water in infants and young children (5).

Analysis of plasma neutralization and fusion inhibition antibody levels following administration of a single dose of the study medication suggests that doses of as little as 1.25 mg of SB 209763 per kg may be associated with some increase in antiviral activity. However, given the relatively low neutralization and fusion inhibition antibody titers that were achieved with the highest dosing regimen, it is not surprising that differences in RSV attack rates were not observed. Also, the high predose neutralization-antibody titer in placebo recipients may have conferred some degree of protection, making it difficult to detect a difference in RSV infection rates between groups. Experience with a polyclonal, hyperimmune RSV globulin indicates that serum neutralizing-antibody titers of greater than 1:200 are necessary to limit RSV replication in the lower respiratory tract. Because of the presence of functionally distinct populations among the RSV-neutralizing antibodies in a polyclonal antibody preparation, only a subset of neutralizing antibodies may actually be protective (14). In this trial, the absence of a cumulative effect on antibody concentration that would be derived from serial doses of SB 209763 also may have contributed to the lack of an effect on RSV infection rates. However, the lack of toxicity and the relative ease of i.m. administration justify further consideration of RSV prophylaxis with a monoclonal antibody.