Abstract
Suramin, at non-cytotoxic doses, reverses chemoresistance and enhances the activity of mitomycin C (MMC) in mice bearing human bladder xenograft tumors. The present study evaluated the pharmacokinetics of the intravesical suramin and MMC, alone or in combination, in dogs. Animals received either high dose suramin (20 mg/ml), low dose suramin (6 mg/ml), MMC (2 mg/ml), or combination of low dose suramin and MMC, instilled for 2 h. The dosing volume was 20 ml. All groups showed dilution of drug levels over time due to continued urine production. For single agent suramin, the results showed (a) 5% to 10% penetration into bladder tissues, (b) minimal and clinically insignificant systemic absorption (i.e., undetectable at low dose or a peak concentration that was 6,000× lower than urine concentrations), and (c) disproportionally higher drug penetration and concentrations in bladder tissues at the higher dose. Results for single agent MMC are consistent with our earlier observations. The co-administration of MMC did not alter the plasma, urine, or tissue pharmacokinetics of suramin. Adding suramin did not alter plasma or tissue pharmacokinetics of MMC, but lowered the MMC concentrations in urine by about 20%. This may be in part due to accelerated MMC degradation by co-incubation of suramin or due to variations in urine production rate (because animals were allowed for water during treatment). Suramin readily penetrates the urothelium and into deeper bladder tissues, indicating its potential utility in intravesical therapy.
Key words: administration, intravesical, mitomycin, suramin, urinary bladder neoplasms
INTRODUCTION
Intravesical chemotherapy is used to reduce the recurrence of non-muscle invading bladder cancer (1,2). We have shown that the variable and incomplete response in patients can be attributed to insufficient drug delivery to tumor cells and tumor resistance to chemotherapeutic agents (3–5). We further showed in a phase III trial that pharmacokinetic interventions can maximize the mitomycin C (MMC) delivery to bladder tissues and improve treatment efficacy (e.g., increasing the 5-year recurrence-free rate from 25% to 41%) (3). Our second strategy is to overcome tumor resistance to chemotherapy, for which we have introduced the concept of using non-cytotoxic doses of suramin as a chemosensitizer, in ex vivo and in vivo preclinical models (6–8). We have further investigated suramin, at doses that yielded the non-cytotoxic plasma concentrations of 10 ~ 50 μM in plasma in four phase I/II trials in chemotherapy-naïve, -pretreated, or -refractory patients with nonsmall cell lung cancer, breast cancer, and renal cell cancer (9–12). These data have led to two randomized trials being initiated in lung cancer (11,12). Additional preclinical data in patient bladder tumor histocultures and mice bearing RT4 transitional cell carcinoma indicate non-cytotoxic suramin enhances the efficacy of MMC (13). Hence, clinical evaluation of combination intravesical therapy with MMC and suramin is of interest.
The results of two phase I trials evaluating intravesical therapy with suramin alone show that it can be safely administered to patients between 100 and 150 mg/ml (14,15). The goal of these earlier trials was to use suramin as the cytotoxic therapeutic. In contrast, we are interested in using suramin as a sensitizer. For this effect, suramin shows an unusual dose response; benefits were observed only at the non-cytotoxic concentrations of 10–50 μM and were abolished at higher concentrations (8,11). Hence, there is a need to define the relationship between the dose and the drug concentrations in bladder tissues for intravesical suramin and to evaluate possible pharmacokinetic interactions between suramin and MMC, prior to embarking on a clinical study of the MMC/suramin combination. This was addressed by the present study. Dogs were used as the animal model due to the similarity of the anatomy and thickness of the dog bladders to human bladders and because intravesical administration can be readily performed in dogs (16–18).
MATERIALS AND METHODS
Chemicals and Reagents
Suramin, trypan blue, tetrabutylammonium bromide, and tetrabutylammonium hydrogensulfate were obtained from Sigma Chemical Co. (St. Louis, Missouri), HPLC grade reagents from Fisher Scientific (Fair Lawn, New Jersey), MMC from Bristol Myer Squibb Co. (Wallingford, Connecticut), and porfiromycin from National Cancer Institute (Bethesda, Maryland). All reagents were used as received.
Animal Protocols
The protocol was approved by the Institutional Animal Care and Use Committee of the Ohio State University. The study used 18 male or female beagle dogs, donated by Battelle Memorial Institute (Columbus, Ohio) and Springborn Laboratory (Springfield, Ohio) and weighing 10.5 ± 1.4 kg (mean ± SD). Animals received either single agent suramin at high dose (20 mg/ml, n = 3) or low dose (6 mg/ml, n = 5), single agent MMC (2 mg/ml, n = 4), or the combination of low dose suramin plus MMC (n = 5). Dosing solutions (20 ml) were prepared in physiologic saline. Procedures were as previously described (18). Briefly, animals were fasted overnight with free access to water. Animal were anesthetized with isofluorane inhalation. Urinary catheters were used for dose instillation and urine sampling and jugular vein catheters for blood sampling. Immediately before treatment, the urinary bladder was emptied and rinsed thrice with normal saline. Animals showing blood in urine were regarded as having a damaged urothelium and were excluded. Drug solution was instilled intravesically for 2 h and the catheters were kept closed during this period. Serial urine and blood samples were obtained. After treatment, the bladder was surgically excised, drained of its content, and cut into three pieces (left and right lateral sides, dome). Tissue sections were snap-frozen in liquid nitrogen. Tissue removal and freezing was completed within 2 min after draining the bladder content or within 5 min after surgical incision of the abdomen wall.
Tissue Sample Processing
Bladder pieces were cryo-sectioned into a series of 20-μm thick slices, parallel to the urothelial surface, as described (18). To avoid drug contamination on tissue sections, the first two section slices were discarded. Extraction of MMC or suramin from bladders after treatments with single agents used previously described methods (12, 16).
For animals treated with both suramin and MMC, tissues were first extracted for MMC. The residual tissues were then extracted for suramin. A pilot study comparing the suramin extraction yield in tissue homogenates with and without a prior extraction for MMC established that these procedures yielded 92–96% recovery of suramin in tissue, indicating near-complete suramin recovery after the sequential extraction procedures.
Other Methodologies
Extraction of suramin or MMC in plasma and urine samples, as well as analysis of drug concentrations by high performance liquid chromatography, were as previously described (12, 16).
MMC Stability in Urine in the Presence Suramin
Suramin (6 mg/ml) and MMC (2 mg/ml) were added to dog urine (collected from dogs not in the study) and incubated at 37°C. Samples were withdrawn and analyzed for pH values and drug concentrations.
Pharmacokinetic Data Analysis
Urine concentration–time profiles and tissue concentration–depth profiles were analyzed using Eqs. 1 and 2, respectively. These equations have been used successfully for several other drugs (16,18).
 |
1 |
 |
2 |
Cu is urine concentration at time t. (ka + kd) is a hybridized constant describing drug adsorption and degradation during treatment. Vu is urine volume at time t and is the sum of the dosing solution volume (V0), residual urine volume at time of dose instillation (Vres), and urine produced during treatment (k0∙t). k0 is the zero order urine production rate. Cx is drug concentration in the capillary-perfused submucosa and muscularis at distance x from the urothelial surface. Concentration decline over the urothelium, from 0 to 50 μm in dog bladders, is presumed to be linear with depth due to absence of capillaries (and hence the first order drug removal by blood flow). Cb is average drug concentration of the blood perfusing the capillary-rich tissue. Half width (W1/2) is the distance by which drug concentration drops 50%. The average tissue concentration (Cave) was calculated as (total drug amount in bladder tissue) divided by (total tissue weight).
Statistical Analysis
Comparison of values between groups was performed using ANOVA for comparisons between three or more groups, or two-tailed Student’s t tests for two groups.
RESULTS
Plasma Pharmacokinetics of Intravesical Single Agent Suramin in Dogs
Animals treated with low dose suramin (6 mg/ml) showed undetectable plasma drug concentrations (<1 μg/ml) at all times. The high dose group showed increasing plasma concentrations to reach the maximum level of 2.53 ± 0.14 μg/ml (mean ± SD) at the end of the 2-h instillation (Fig. 1). This concentration was >6,000× lower compared to the urine concentration at 2 h (Fig. 2).
Fig. 1.
Plasma concentrations of suramin and MMC during intravesical treatment. Dogs received suramin, MMC, or both, by intravesical installation. Animals received single agent suramin at two doses, low dose (120 mg/20 ml or 6 mg/ml, n = 5) or high dose (400 mg/20 ml or 20 mg/ml, n = 3); single agent MMC (40 mg/20 ml, n = 4) or combination of low dose suramin and MMC (n = 5). Data are mean and 1 SD. Plasma samples collected at 5 min in the single agent high dose suramin group, and all time points in treatments using the low dose suramin had undetectable suramin concentrations (i.e., below the detection limit of 1 μg/ml). Note the logarithmic scale
Fig. 2.
Urine concentration–time profiles of suramin and MMC during intravesical treatments. Dogs received suramin, MMC, or both, by intravesical installation. Animals received single agent suramin at two doses, low dose (120 mg/20 ml or 6 mg/ml, n = 5) or high dose (400 mg/20 ml or 20 mg/ml, n = 3); single agent MMC (40 mg/20 ml, n = 4) or combination of low dose suramin and MMC (n = 5). Data are mean and 1 SD. Note the linear scale
Urine Pharmacokinetics of Intravesical Single Agent Suramin in Dogs
Urine concentrations declined with time continuously during the 2-h experiment, due to urine production (Fig. 2). Table I summarizes the urine pharmacokinetic parameters. Compared to the high dose group, the increase in urine volume was higher in the low dose group, i.e., 50% vs 12%, which corresponded to the greater dilution of urine concentration in the low dose group (58% vs 80% of the initial concentrations in the dosing solutions). The reasons are unclear. The dose fractions recovered in the 2-h urine were 90.7 ± 1.6% in the low dose group and 95.7 ± 6.3%, in the high dose group, indicating less than 10% of the dose was degraded or absorbed.
Table I.
Urine Pharmacokinetics of Suramin and MMC
| Groups | C u (μg/ml) | V res (ml) | k 0 (ml/min × 10−2) | k a + k d (min−1 × 10−4) | V final (ml) | IUV (%) | |
|---|---|---|---|---|---|---|---|
| A. Suramin | |||||||
| Group 1: High dose suramin (n = 3) | Range | 15.6–17.4 | 1.12–1.66 | 0.73–2.85 | 0–2.38 | 22–25 | 4.1–15.8 |
| Median | 16.5 | 1.59 | 2.79 | 0.0001 | 25 | 15.4 | |
| Mean ± SD | 16.5 ± 0.89* | 1.46 ± 0.29 | 2.12 ± 1.21 | 0.79 ± 1.37** | 24 ± 2 | 11.8 ± 6.6 | |
| Group 2: Low dose suramin without MMC (n = 5) | Range | 2.27–3.74 | 0.32–5.33 | 3.89–17.2 | 5.34–22.0 | 30–45 | 18.4–85.1 |
| Median | 3.28 | 2.51 | 8.90 | 10.70 | 31 | 52.5 | |
| Mean ± SD | 3.07 ± 0.68 | 2.78 ± 2.05 | 9.52 ± 4.95 | 12.8 ± 6.5 | 34 ± 6 | 50.3 ± 24.3 | |
| Group 3: Low dose suramin with MMC (suramin, n = 5) | Range | 3.10–4.44 | 2.18–3.96 | 2.35–10.7 | 2.85–19.1 | 25–35 | 12.7–57.8 |
| Median | 3.41 | 3.66 | 9.20 | 8.30 | 35 | 46.1 | |
| Mean ± SD | 3.60 ± 0.51 | 3.17 ± 0.91 | 7.35 ± 3.50 | 8.82 ± 6.47 | 32 ± 5 | 38.1 ± 18.4 | |
| B. MMC | |||||||
| Group 4: MMC without low dose suramin (n = 4) | Range | 1.04–1.54 | 0–3.04 | 6.62–11.3 | 0.47–27.2 | 25–35 | 8.5–62.9 |
| Median | 1.19 | 1.77 | 7.48 | 22.0 | 30 | 43.0 | |
| Mean ± SD | 1.24 ± 0.23 | 1.64 ± 1.27 | 6.96 ± 4.03 | 15.9 ± 11.6 | 30 ± 4 | 39.4 ± 23.3 | |
| Group 3: MMC with low dose suramin (n = 5) | Range | 0.76–1.30 | 2.18–3.96 | 2.35–10.7 | 3.56–21.4 | 25–35 | 12.7–57.8 |
| Median | 0.82 | 3.66 | 9.20 | 16.7 | 35 | 46.1 | |
| Mean ± SD | 0.96 ± 0.24 | 3.17 ± 0.91a | 7.35 ± 3.50a | 14.8 ± 6.9 | 32 ± 5 | 38.1 ± 18.4 | |
Dogs received intravesical treatment as described in the legend of Fig. 1. The urine concentration–time profiles were analyzed using Eq. 1. Differences between group means were evaluated by ANOVA for comparisons between all groups (V res, k 0, V final) or for more than two groups (k a + k d for suramin). Comparisons of sets of two groups used Student’s two-tailed t test. Groups that show significant differences are noted by asterisks
C u urine concentration at 120 min, V res residual volume of urine at the time of dosing, k 0 the urine production rate, k a + k d the first order rate constant for drug absorption into the bladder and degradation in urine, V final total urine volume at 120 min, IUV increase in urine volume during treatment
*p < 0.05, C u for Group 1 was higher than for Groups 2 and 3 (ANOVA with post hoc Tukey test). This is expected due to the higher dose. **p < 0.05, k a + k d for Group 1 was lower than for Group 2 and 3 (ANOVA with post hoc Tukey test)
aGroup 3: values for V res and k 0 were calculated by fitting of suramin concentration data
Bladder Tissue Pharmacokinetics of Intravesical Single Agent Suramin in Dogs
Figure 3 shows the tissue concentration–depth profiles of suramin, and Table II summarizes the tissue pharmacokinetic parameters. The half width of tissue concentration decline was about 300 μm. The ratio of urothelial to urine drug concentration (i.e., Curo/Cu ratio), which gives a measure of the partition of suramin across the urothelium, was significantly higher in the high dose group compared to the low dose group (~10% vs ~5%, p < 0.05). The same trend was observed for the ratio between the average tissue concentration and the urine concentration (i.e., Cave/Cu ratio, which is a measure of the extent of tissue penetration). Together, these data indicate disproportionately higher penetration and absorption of suramin into the bladder wall at higher urine concentrations. The lower absorption of the low dose is in agreement with the undetectable plasma concentrations in this group (i.e., <1 μg/ml). No differences were observed in the other tissue pharmacokinetic parameters.
Fig. 3.
Tissue–depth profiles of suramin and MMC. Dogs received suramin, MMC, or both, by intravesical installation. Animals received single agent suramin at two doses, low dose (120 mg/20 ml or 6 mg/ml, n = 5) or high dose (400 mg/20 ml or 20/ml, n = 3); single agent MMC (40 mg/20 ml, n = 4) or combination of low dose suramin and MMC (n = 5). Data are mean and 1 SD. For MMC and low dose suramin, several animals showed undetectable levels at depth beyond 1,200 μm. Hence, the profiles were limited to the first 1,200 μm. Note the logarithmic scale
Table II.
Tissue Pharmacokinetic Parameters of Suramin and MMC
| Groups | C uro (μg/g) | C ave (μg/g) | C b (μg/g) | C uro/C u (%) | C ave/C u (%) | W 1/2 (μm) | |
|---|---|---|---|---|---|---|---|
| A. Suramin | |||||||
| Group 1: High dose suramin (n = 3) | Range | 1,222–2,684 | 90.1–506 | 0–35.1 | 7.30–15.4 | 0.58–4.33 | 250–728 |
| Median | 1,525.8 | 451 | 0.849 | 9.75 | 2.82 | 320 | |
| Mean ± SD | 1,811 ± 772* | 349 ± 226* | 12.0 ± 20.0 | 10.8 ± 4.2 | 2.58 ± 1.89 | 432 ± 258 | |
| Group 2: Low dose suramin without MMC (n = 5) | Range | 35.4–641 | 23.7–73.9 | 0–7.86 | 1.91–17.1 | 0.86–2.23 | 110–643 |
| Median | 97.9 | 37.6 | 0 | 3.94 | 1.21 | 316 | |
| Mean ± SD | 166 ± 174 | 41.6 ± 15.6 | 1.84 ± 2.67 | 5.09 ± 4.40 | 1.44 ± 0.55 | 316 ± 152 | |
| Group 3: Low dose suramin with MMC (n = 5) | Range | 56.1–354 | 24.8–113.2 | 0–10.4 | 1.66–9.65 | 0.54–2.55 | 100–1,097 |
| Median | 115 | 44.8 | 3.56 | 3.10 | 1.33 | 287 | |
| Mean ± SD | 147 ± 87 | 50.1 ± 23.8 | 3.60 ± 0.47 | 4.01 ± 2.19 | 1.40 ± 0.60 | 380 ± 264 | |
| B. MMC | |||||||
| Group 4: MMC without low dose suramin (n = 4) | Range | 8.53–82.9 | 2.48–25.8 | 0–2.71 | 0.81–7.96 | 0.19–2.48 | 100–342 |
| Median | 31.6 | 5.66 | 0.40 | 3.17 | 0.45 | 207 | |
| Mean ± SD | 37.2 ± 27.9 | 6.95 ± 5.76 | 0.74 ± 0.79 | 3.27 ± 2.27 | 0.65 ± 0.63 | 216 ± 91 | |
| Group 3: MMC with low dose suramin (n = 5) | Range | 9.67–72.2 | 3.14–17.4 | 0.09–6.34 | 0.74–9.41 | 0.24–2.13 | 100–297 |
| Median | 30.9 | 5.97 | 1.11 | 3.48 | 0.59 | 162 | |
| Mean ± SD | 38.1 ± 20.7 | 7.00 ± 4.52 | 1.68 ± 1.70 | 4.30 ± 2.73 | 0.77 ± 0.51 | 178 ± 74 | |
Parameters were calculated using Eq. 2. Differences between group means were evaluated by ANOVA for comparisons between more than two groups (Groups 1, 2, and 3 for suramin). Comparisons of sets of two groups used Student’s two-tailed t test. Groups that show significant differences are noted by asterisks
C ave average tissue concentration, C uro drug concentration in the urothelium (at 50 μm depth), C b drug concentration in the deep muscle layer, C u final urine concentration, W 1/2 (half width) tissue distance over which C declines by 50%
*p < 0.05, C uro and C ave for Group 1 were higher than for Groups 2 and 3 (ANOVA with post hoc Tukey test). This is expected due to the higher dose
Plasma, Urine, and Bladder Tissue Pharmacokinetics of Intravesical MMC in Dogs
Figures 1, 2, and 3 show the plasma, urine, and bladder tissue concentrations, respectively, after intravesical MMC. The pharmacokinetic parameters are summarized in Table II. Plasma concentrations reached an early maximum at approximately 15 min, followed by essentially steady concentrations. Urine concentrations show an initial drop from the dosing solution concentration of 2 mg/ml due to dilution in the residual urine, followed by a slow concentration decline due to dilution in the newly formed urine, and drug absorption and degradation. Tissue concentrations declined approximately 30-fold over the urothelium (Curo/Cu ~3.3%), followed by a log-linear decline with depth as previously observed [16]. The steepness of the decline was greater than previously observed [16]; the reasons are not apparent.
Potential Pharmacokinetic Interactions Between Intravesical Suramin and Intravesical MMC
The results are shown in Figs. 1, 2, and 3 and Tables I and II.
For suramin, there were no differences in the urine and tissue levels in the absence or presence of MMC. Similarly, the plasma suramin concentration remained below the detection limit of 1 μg/ml, with or without MMC. These results indicate that addition of MMC did not significantly alter the urine and plasma pharmacokinetics of suramin.
For MMC, the addition of suramin produced a relative minor, but statistically significant, decrease in the recovery of MMC in urine at 2 h (from 87.4 ± 6.8% to 78.9 ± 6.6%, p < 0.05). This difference was also reflected in the urine concentration–time profiles with slightly lower concentrations in the combination group, partly explained by differences in residual volume and urine production rate. Urine pH was measured in some of the dogs, but did not show differences between groups. This small difference in urine concentration–time profiles did not affect the MMC absorption into tissues or blood, as indicated by the nearly superimposable plasma and tissue concentration–depth profiles for single agent MMC or MMC/suramin combination.
MMC Stability in Suramin-Containing Dog Urine
To investigate MMC degradation as possible cause of the lower urine concentrations in the presence of suramin, we determined the concentration decline and changes in urine pH because we have shown that MMC is unstable in acidic urine (19). After incubation of the suramin MMC combination in urine for 2 h in vitro, the urine pH decreased from 7.0 to 6.2 and the amount of MMC declined to 91.7 ± 4.10% (n = 5) of the initial amount.
DISCUSSION
The goals of the present study were to evaluate the ability of suramin to penetrate bladder tissue during intravesical therapy and the extent of such penetration and to evaluate the feasibility of using suramin as a sensitizer with MMC. The major results are as follows.
Intravesical Suramin Yields Appreciable Concentrations in Bladder Tissues
Urothelium is the major barrier to drug absorption into the bladder wall. Compounds with high lipophilicity (e.g., paclitaxel with an octanol/water partition coefficient >99) show more favorable urothelial penetration compared to compounds with lower lipophilicity (e.g., MMC, doxorubicin and 5-fluorouridine, with octanol/water partition coefficients of #0.5) (20). This observation is consistent with passive diffusion as the major transport mechanism through the urothelium. Suramin, a polysulfonated naphthylurea with six sulfonate groups, is highly polar with a lower lipophilicity (octanol/water partition coefficient is 0.00032 (21) and, hence, would be expected to have low penetration across the urothelium. Surprisingly, the results of the present study indicate appreciable drug penetration; the respective Curo/Cu ratio, Curo (concentration in the urothelium), and Cave (average bladder tissue concentration) were ~5%, ~170 μg/g (~120 μM), and ~40 μg/g (~30 μM) at the 6 mg/ml dose and ~10%, ~1,700 μg/g (~1,200 μM), and ~420 μg/g (~300 μM) at the 20 mg/ml dose. These results indicate a nonlinear, dose-dependent drug penetration, with disproportionately higher penetration at the higher dose. In addition, the Curo/Cu ratio for suramin was two to three times higher compared to MMC, doxorubicin, and 5-fluorouridine (5–10% vs ~3%) (16,18,22).
Two possible reasons for the higher bladder tissue penetration of suramin are differences in transport mechanism(s) and drug retention in tissues. With respect to transport, we reported a linear, dose-independent increase for MMC, consistent with passive diffusion as the primary transport mechanism (16). Additional facilitated or active transport mechanisms would increase the penetration across the urothelium. For example, suramin can enter cells by endocytosis or pinocytosis (23). The same in urothelial cells would explain the higher penetration. Of interest is the observation of the significant increase in suramin absorption with dose (i.e., 2× higher Curo/Cu when the dose was increased from 6 mg/ml to 20 mg/ml). In contrast, the penetration of MMC was not altered when the dose was increased from 1 to 2 mg/ml (16). The reasons for the dose-dependent urothelial penetration by suramin are not apparent.
With respect to retention, we reported that drugs with insignificant tissue binding such as MMC rapidly equilibrate with bladder tissues and reach steady state equilibrium within minutes (18), whereas drugs such as paclitaxel that are tightly bound to tissues or proteins are retained in bladder tissues (20, 24). Suramin is highly protein-bound to plasma proteins (>99.7%), with albumin accounting for 80–90% of the total binding proteins (25,26). Based on the high albumin concentration in interstitial fluid (~80% of that in plasma (27), high protein-binding of suramin is expected and would explain the higher Curo/Cu ratios.
Applying Bladder Tissue Pharmacokinetics of Intravesical Suramin to Calculate the Tissue Levels for Previous Treatments in Patients
Two phase I clinical trials have employed intravesical doses of 100–150 mg/ml suramin as a single agent cytotoxic (1,2,14,15). To evaluate whether these doses deliver sufficient drug levels to cause cytotoxicity, we used the bladder tissue pharmacokinetic parameters established in the current study (for the 20 mg/ml dose) to estimate the corresponding tissue concentrations. Assuming no further increases in the Curo/Cu ratios at the higher doses used in humans, we calculated, at the time of dose instillation, the Curo would be ~7 mM and ~10 mM and the Cave would be ~2 and ~2.5 mM, for the 100 and 150 mg/ml dose, respectively. These levels are ~10-folds higher than the 175–280 μM concentrations that cause cytotoxicity in cultured cells (28).
Pharmacokinetic Interactions Between Intravesical MMC and Suramin
The present results show that MMC did not alter the plasma, urine, or bladder tissue pharmacokinetics of intravesical suramin. Adding suramin to MMC decreased the urine concentrations and dose fraction of MMC remaining at 2 h. However, these changes, while statistically significant, were relatively minor (<10%) and did not result in significant changes in the tissue pharmacokinetics of MMC or its absorption into the systemic circulation.
Potential Use of Intravesical Suramin to Enhance the Activity of Intravesical MMC
Activity of suramin as a chemosensitizer is observed at extracellular concentrations between 10 and 50 μM and is abolished at higher concentrations. Based on the results of the present study, the low dose of 6 mg/ml yielded 100 μM in the urothelium (50 μm depth) and average concentration of 35 μM in bladder tissue. Hence, clinical evaluation of suramin as a sensitizer in intravesical therapy should use starting doses lower than 6 mg/ml, e.g., 3 mg/ml. An added advantage of using these low doses is the minimal suramin systemic exposure of less than 1 μg/ml. Finally, in view of the long retention of suramin in tissues, e.g., up to >12 weeks in rats (29), a routine weekly dose schedule would cause drug accumulation. Additional studies to address the post-treatment suramin retention in bladder tissues and the dosing frequency are warranted.
Conclusions
Upon intravesical instillation, suramin penetrated the urothelium and yielded appreciable drug concentrations in bladder tissues, in a dose-dependent manner (increasing with higher doses). There are no or minimal pharmacokinetic interactions between suramin and MMC, indicating the feasibility of combining intravesical suramin with intravesical MMC.
Acknowledgments
This work was supported in part by research grants R01CA93871 and R37CA49816 from NCI, NIH, and DHHS.
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