Intra-arterial bromodeoxyuridine radiosensitization and radiation in treatment of malignant astrocytomas

A systematic review on intra-arterial cerebral infusions of chemotherapeutics in the treatment of glioblastoma multiforme: The state-of-the-art

Objective

To provide a comprehensive review of intra-arterial cerebral infusions of chemotherapeutics in glioblastoma multiforme treatment and discuss potential research aims. We describe technical aspects of the intra-arterial delivery, methods of blood-brain barrier disruption, the role of intraoperative imaging and clinical trials involving intra-arterial cerebral infusions of chemotherapeutics in the treatment of glioblastoma multiforme.

Method

159 articles in English were reviewed and used as the foundation for this paper. The Medline/Pubmed, Cochrane databases, Google Scholar, Scielo and PEDro databases have been used to select the most relevant and influential papers on the intra-arterial cerebral infusions of chemotherapeutics in the treatment of glioblastoma multiforme. Additionally, we have included some relevant clinical trials involving intra-arterial delivery of chemotherapeutics to other than GBM brain tumours.

Conclusion

Considering that conventional treatments for glioblastoma multiforme fall short of providing a significant therapeutic benefit, with a majority of patients relapsing, the neuro-oncological community has considered intra-arterial administration of chemotherapeutics as an alternative to oral or intravenous administration. Numerous studies have proven the safety of IA delivery of chemotherapy and its ability to ensure higher drug concentrations in targeted areas, simultaneously limiting systemic toxicity. Nonetheless, the scarcity of phase III trials prevents any declaration of a therapeutic benefit. Given that the likelihood of a single therapeutic agent which will be effective for the treatment of glioblastoma multiforme is extremely low, it is paramount to establish an adequate multimodal therapy which will have a synergistic effect on the diverse pathogenesis of GBM. Precise quantitative and spatial monitoring is necessary to guarantee the accurate delivery of the therapeutic to the tumour. New and comprehensive pharmacokinetic models, a more elaborate understanding of glioblastoma biology and effective methods of diminishing treatment-related neurotoxicity are paramount for intra-arterial cerebral infusion of chemotherapeutics to become a mainstay treatment for glioblastoma multiforme. Additional use of other imaging methods like MRI guidance during the procedure could have an edge over X-ray alone and aid in selecting proper arteries as well as infusion parameters of chemotherapeutics making the procedure safer and more effective.

The use of radiosensitizing agents in the therapy of glioblastoma multiforme-a comprehensive review

BACKGROUND

Glioblastoma is the most common malignant brain tumor in human adults. Despite several improvements in resective as well as adjuvant therapy over the last decades, its overall prognosis remains poor. As a means of improving patient outcome, the possibility of enhancing radiation response by using radiosensitizing agents has been tested in an array of studies.

METHODS

A comprehensive review of clinical trials involving radiation therapy in combination with radiosensitizing agents on patients diagnosed with glioblastoma was performed in the National Center for Biotechnology Information's PubMed database.

RESULTS

A total of 96 papers addressing this matter were published between 1976 and 2021, of which 63 matched the subject of this paper. All papers were reviewed, and their findings discussed in the context of their underlining mechanisms of radiosensitization.

CONCLUSION

In the history of glioblastoma treatment, several approaches of optimizing radiation-effectiveness using radiosensitizers have been made. Even though several different strategies and agents have been explored, clear evidence of improved patient outcome is still missing. Tissue-selectiveness and penetration of the blood-brain barrier seem to be major roadblocks; nevertheless, modern strategies try to circumvent these obstacles, using novel sensitizers based on preclinical data or alternative ways of delivery.

Strategies for Improved Intra-arterial Treatments Targeting Brain Tumors: a Systematic Review

Conventional treatments for brain tumors relying on surgery, radiation, and systemic chemotherapy are often associated with high recurrence and poor prognosis. In recent decades, intra-arterial administration of anti-cancer drugs has been considered a suitable alternative drug delivery route to intravenous and oral administration. Intra-arterial administration is believed to offer increasing drug responses by primary and metastatic brain tumors, and to be associated with better median overall survival. By directly injecting therapeutic agents into carotid or vertebral artery, intra-arterial administration rapidly increases intra-tumoral drug concentration but lowers systemic exposure. However, unexpected vascular or neural toxicity has questioned the therapeutic safety of intra-arterial drug administration and limits its widespread clinical application. Therefore, improving targeting and accuracy of intra-arterial administration has become a major research focus. This systematic review categorizes strategies for optimizing intra-arterial administration into five categories: (1) transient blood-brain barrier (BBB)/blood-tumor barrier (BTB) disruption, (2) regional cerebral hypoperfusion for peritumoral hemodynamic changes, (3) superselective endovascular intervention, (4) high-resolution imaging techniques, and (5) others such as cell and gene therapy. We summarize and discuss both preclinical and clinical research, focusing on advantages and disadvantages of different treatment strategies for a variety of cerebral tumor types.

Erythema multiforme, Stevens Johnson syndrome, and toxic epidermal necrolysis syndrome in patients undergoing radiation therapy: a literature review

OBJECTIVE

Erythema multiforme (EM), Stevens Johnson syndrome (SJS), and toxic epidermal necrolysis syndrome (TENS) are exfoliative disorders that may present as complications in some patients undergoing radiotherapy. The purpose of this literature review was to determine the reported frequency of these exanthemata in irradiated patients.

METHODS

A comprehensive search from 1903 to 2011, identified 89 articles with 165 cases.

RESULTS

Of 151 evaluable cases, 57 (38%) described EM; 46 (30.5%) SJS; 14 (9%) SJS/TENS overlap; and 34 (22.5%) TENS. Ninety-three percent underwent radiotherapy for either a primary or metastatic malignancy. A majority of patients were simultaneously treated with medications known to precipitate these exanthemata. Of the 61 patients receiving antiepileptic medications during radiotherapy, 48 were treated prophylactically and 13 for seizures, most frequently with phenytoin or phenobarbital. Amifostine was the second most common medication associated with radiotherapy and these reactions. Fourteen (23%) patients on anticonvulsant medication, and 11 (38%) on other medications died compared with 3 (8%) patients treated with radiotherapy alone (P = 0.002). No deaths occurred among irradiated patients receiving amifostine.

CONCLUSIONS

EM, SJS, and TENS were rarely reported in patients undergoing radiotherapy alone. The majority of SJS and TENS occurred in irradiated patients with concomitant prescribed medications.

Targeted endoradiotherapy using nucleotides

Increased cellular proliferation is an integral part of the cancer phenotype. Hence, the sustained and continued demand on supply of DNA building blocks during the DNA replication presents a potential target for therapeutic intervention. For this propose, the α and Auger electron emitting nucleotides analogs are attractive for targeted endoradiotherapy, given that DNA of malignant cells is selectively addressed. This review summarizes development and preclinical and clinical studies of endoradiotherapeutic acting nucleoside analogs with a special focus on thymidine analogs.

Determination of hypersensitivity to genotoxic agents among Escherichia coli single gene knockout mutants

We have tested the KEIO collection of 3985 different viable single gene knockouts in Escherichia coli to identify genes whose loss increases sensitivity to one or more of six different chemotherapeutic agents and mutagens: Bleomycin (BLM), Cisplatin (CPT), ICR-191 (ICR), 5-azacytidine (5AZ), Zebularine (ZEB), and 5-bromo-2'-deoxyuridine (5BdU). We discovered a set of 156 strains that display a significant increase in sensitivity to at least one of the agents tested. Each genotoxic agent generates a distinct "sensitivity profile" that is characteristic of the agent. Comparison with an independent study of sensitivity profiles for an extensive set of antibiotics pinpoints those effects that are relatively specific for each agent. In some cases engineered double mutants have greatly increased effects. These results provide insight into the mechanism of action of each agent, and define targets for the design of co-drugs that can potentiate these agents. An example is the finding that mutants lacking one of several genes in the folate biosynthetic pathway are hypersensitive to ZEB, leading to a demonstration of synergy between trimethoprim and ZEB.

On the safety of 5-[125I]iodo-2'-deoxyuridine--Preclinical evaluation in swine

To increase tumor incorporation and minimize hepatic degradation of radio-IUdR, compartmental administration routes are being considered as an alternative to intravenous (i.v.) injections. Although there are significant data on the biodistribution and some reports on radiotoxicity of i.v.-administered 125IUdR, similar results for other routes of delivery are not available. We have undertaken a series of experiments intended to examine radiation effects of 125IUdR after intravesical (3 swine; eight 3 mCi doses at 4-day intervals), intracarotid (3 swine; two 10 mCi doses at 2-week intervals), and intra-aortic (5 swine, single dose of 10 mCi) administration in a swine model. Liver, renal functions, and complete blood counts were monitored throughout the duration of the experiment. Pharmacokinetics, systemic distribution of radioactivity and metabolites were measured. The normal tissue 125IUdR uptake and histology were determined after necropsy. No adverse systemic effects were identified. Clinical observations, laboratory data, and necropsy results were within normal range.

Radiation therapy and bromodeoxyuridine chemotherapy followed by procarbazine, lomustine, and vincristine for the treatment of anaplastic gliomas

PURPOSE

To conduct a Phase II study to evaluate the long-term efficacy and safety of radiotherapy combined with intravenous bromodeoxyuridine for patients with anaplastic glioma tumors.

METHODS AND MATERIALS

Between 1983 and 1987, study patients received 1.7-1.8 Gy radiation once a day, Monday through Friday, to a total dose of 60 Gy. On the Thursday prior to beginning radiotherapy and for the next 5 weeks (6 weeks total), patients received a continuous 96 h intravenous infusion of bromodeoxyuridine at 0.8 g/m2/24 h; following radiotherapy, patients received procarbazine, lomustine (CCNU), and vincristine (PCV) for 1 year or until tumor progressed.

RESULTS

One-hundred thirty eight patients (median age, 43 years) were evaluable for analysis. Estimated 4-year survival for the anaplastic astrocytoma (AA) stratum (n = 116) is 46%. For the astrocytoma (ASTRO) stratum (n = 22), the 6-year survival is estimated at 79%. Estimated 4-year progression-free survival for AAs is 42%, and for ASTROs, 68%. Whole brain irradiation was used in 23% and limited-field irradiation in 77%; patients receiving limited-field irradiation had a better survival rate (p = 0.07). Total tumor resection was performed in 15%, partial resection in 53%, and biopsy only in 32%. For the 81 patients with tumor recurrence, 34 (42%) are known to have received additional treatment(s). For AA, fits of the Cox proportional hazards regression model showed that covariates individually predictive of survival were younger age (p < 0.001), Karnofsky performance score (p = 0.10). Major toxicities were rash during Weeks 1 through 6 requiring dose modification in 14%, Grade > or = III leukopenia in 18%, and Grade > or = III thrombocytopeni in 9%.

CONCLUSION

The study suggests that the bromodeoxyuridine-radiotherapy-PCV, compared with other published therapies, can improve progression-free survival, and aggressive treatment of ASTRO patients can lead to substantial increases in survival compared to published survival data.

Bromodeoxyuridine-mediated radiosensitization in human glioma: the effect of concentration, duration, and fluoropyrimidine modulation

PURPOSE

To define the relative influence of duration of exposure, concentration, and modulation by fluorodeoxyuridines (FdUrd) on the incorporation of 5-bromo-2-deoxyuridine (BrdUrd) into DNA of a human malignant glioma line (D-54) in vitro and in vivo.

MATERIALS AND METHODS

IN VITRO STUDIES

an established human malignant glioma line (D-54) was exposed to a clinically achievable concentration of BrdUrd to model intravenous (1 microM BrdUrd) and intraarterial (4 microM BrdUrd) conditions. The influence of modulation was assessed using 1 nM FdUrd. Incorporation of BrdUrd, radiosensitization, and cytotoxicity were determined after 24, 72, and 120 h drug exposures. In Vivo studies: nude mice bearing D-54 xenografts were infused with BrdUrd at 100 mg/kg/day for 7 and 14 days or BrdUrd at 400 mg/kg/day for 5 days. The influence of modulation was assessed by combining 100 mg/kg/day of BrdUrd with 0.1, 0.3, and 1 mg/kg/day FdUrd for 7 days. Incorporation of BrdUrd into the DNA of tumor, gut, and marrow were determined.

RESULTS

In Vitro: thymidine replacement and radiosensitization were a function of concentration, and incorporation began to plateau after 2 to 3 population doublings. Modulation with 1 nM FdUrd significantly increased incorporation. Radiosensitization was a linear function of thymidine replacement under all conditions tested. In Vivo: infusion with 400 mg/kg/day for 5 days resulted in greater tumor incorporation (10.3 +/- 0.4% thymidine replaced) than treatment with 100 mg/kg/day for 14 days (6.0 +/- 0.6% of thymidine replaced). Infusion of FdUrd with BrdUrd increased normal tissue incorporation of BrdUrd, but failed to increase BrdUrd incorporation in tumor cells.

CONCLUSION

These results suggest that relatively short, high dose rate infusions may be preferable to long, low dose rate infusions. The potential benefit of FdUrd modulation demonstrated in vitro may be difficult to realize using continuous systemic infusions.

The application of 5-bromodeoxyuridine in the management of CNS tumors

A variety of clinical reports have described the application of the bromodeoxyuridine labeling index as an adjunct to conventional pathological examination of CNS tumors. This index has proven useful in predicting the clinical outcome associated with many such tumors. Furthermore, because of its efficacy as a radiosensitizing agent, bromodeoxyuridine (and the closely related iododeoxyuridine) has been used in combination with radiation therapy for malignant glial neoplasms, with some encouraging results. Although most studies suggest that bromodeoxyuridine is safe, there is evidence that this compound does have potential side-effects, including the observation that it is a mutagen and carcinogen in some experimental systems. A number of new alternative approaches for predicting the clinical outcome of CNS tumors has been developed based on an increased understanding of their molecular biology. However, until such approaches are better characterized, the clinical application of bromodeoxyuridine will continue to play an important role in predicting the clinical behavior of many CNS tumors.

Halogenated pyrimidine sensitization of low dose rate irradiation in human malignant glioma

PURPOSE

To determine the potential advantage of combining halogenated pyrimidine radiosensitization and continuous low dose rate irradiation in human malignant glioma.

METHODS AND MATERIALS

An established glioma line (U-251) was incubated with 5-bromo-2-doxyuridine (BrdUrd) at clinically achievable concentrations at three dose rates of interest--100 cGy/min (typical of external beam therapy), 43 cGy/hr (typical of temporary afterloaded implants), and 12 cGy/hr (typical of permanent implants).

RESULTS

After exposure to 1 microM BrdUrd, the greatest enhancement ratio was seen at the 12 cGy/hr dose rate, implying a BrdUrd induced inverse dose rate effect independent of a G2M block. Under these conditions, the mean inactivation dose after 1 microM BrdUrd exposure was equivalent for 100 cGy/min and 12 cGy/hr.

CONCLUSION

These results support the use of halopyrimidines as sensitizers of temporary afterloaded and permanent implants.

Malignant astrocytomas: focal tumor recurrence after focal external beam radiation therapy

Hochberg and Pruitt have reported glioblastomas recurring within 2 cm of the primary site in 90% of patients after whole-brain radiation therapy. They suggested that computerized tomography (CT) scan accuracy would permit smaller radiation fields. A treatment protocol with smaller-field focal brain irradiation following surgical resection is reported. The first 4500 cGy of radiation is focused to within a 3-cm margin around the tumor, with a 1500-cGy boost within a 1.5-cm margin. Forty-two patients with grade III or IV astrocytoma, treated with focal brain radiation therapy were reviewed retrospectively to assess patterns of tumor recurrence. Thirty patients received intra-arterial bromodeoxyuridine (BUdR) radiosensitization with focal brain radiation therapy, and 12 patients underwent conventional focal brain radiation therapy. Tumor margin was defined on preoperative and recurrence CT scans as the contrast-enhanced area; these were traced on acetate templates and compared with each other and with the actual scans. In all 42 patients, the lesion recurred within a 2-cm margin of the original tumor. Four patients had two recurrent areas: the second area was within the 2-cm margin in two, and outside this margin in two. These results are similar to those of Hochberg and Pruitt. It is suggested that focal irradiation is now the optimal treatment for malignant astrocytoma. Since recurrences continue to be within the irradiated volumes, it appears that higher focal doses of radiation are appropriate for clinical treatment trials of malignant astrocytomas.

Cell proliferation in serial biopsies through human malignant brain tumours: measurement using Ki67 antibody labelling

Cell proliferation was assessed in brain tumours using the monoclonal antibody Ki67 which recognizes a nuclear antigen expressed by proliferating cells. Using a novel stereotactic biopsy procedure, serial 1 cm biopsies were taken along a trajectory through six malignant brain tumours. Specimens were also obtained from 10 other brain tumours during conventional surgery. The percentage of Ki67 positive cells was determined as a fraction of the total number of tumour cells present. The Ki67 index for anaplastic astrocytomas and glioblastomas was significantly higher (Ki67 index range 11-18%) than that for benign or low grade tumours. Significant variation in proliferation was measured along the biopsy track through individual tumours (e.g. 0-12.3%) which correlated well with histological appearance. The Ki67 indices of normal brain were very low. In general the Ki67 indices increased with increasing histological grade and also appear to be a useful indicator of the active tumour volume and margin. This method provides spatial information about tumour proliferation which may be used to decide between different treatments and relate to prognosis.

The dependence of halogenated pyrimidine incorporation and radiosensitization on the duration of drug exposure

The influence of the duration of exposure to the halogenated pyrimidines iododeoxyuridine (IdUrd) and bromodeoxyuridine (BrdUrd) on incorporation into DNA and the resulting radiosensitization was studied in cultured human colon cancer cells. Cells were incubated with either 10 microM BrdUrd or IdUrd for periods up to 7 days. They were also assessed for up to 4 days after removal of drug from the medium. Replacement of thymidine by fraudulent bases was measured using a sensitive gas chromatographic, mass spectrometric (GC/MS) assay. Incorporation of BrdUrd and IdUrd plateaued at 35% and 30%, respectively, after 4 days of exposure. Prolonging the time of exposure to 7 days increased cytotoxicity without affecting either incorporation or radiosensitization. Incorporation remained constant for 1-2 days after removal of drug from the medium. Radiosensitization was linearly related to incorporation throughout the range of conditions assessed. These data suggest that it may be possible to develop a predictive assay for radiosensitization based on measurements of halogenated pyrimidine incorporation in a tumor biopsy specimen. They also suggest that a clinical approach based on repeated short exposures to halogenated pyrimidines may present certain advantages over the current practice of prolonged continuous exposure. A Phase I/II trial using IdUrd and external beam irradiation for the treatment of patients with poor prognosis soft tissue sarcomas has been initiated based on this concept.

Critical appraisal of experimental radiation modalities for malignant astrocytomas

The management of patients with supratentorial malignant astrocytomas has remained a major problem. Patients continue to die from a lack of local control in 90% of cases despite an improvement of median survival seen with the use of postoperative radiation therapy. Because of this, there has been considerable interest in exploring novel ways of possibly improving results. This paper reviews the rationale and clinical results with the use of altered fractionation schemes, brachytherapy, radiation sensitizers, hyperthermia, particle therapy, and radiosurgery in the treatment of these patients. Currently, there is no demonstrated advantage with the use of these experimental modalities in the initial management of patients. There would appear to be some benefit for selected patients who are treated with brachytherapy at recurrence, but its efficacy as part of initial management remains to be determined determined in ongoing randomized prospective trials.

Supratentorial malignant glioma: an analysis of radiation therapy in 178 cases

To analyze treatment results of supratentorial malignant gliomas in the megavoltage era, all the histologic specimens were reviewed and glioblastoma multiforme (GBM) was distinguished from anaplastic astrocytoma (AA) by the presence of necrosis. Among those who had completed radiotherapy and who had been followed for at least one year, 135 GBM and 43 AA patients were found. The median survival time (MST) after operation was 12 months for GBM and 18 months for AA. The 5-year survival rate was 0.9% for GBM and 18% for AA. The size of radiation field had little influence on survival time; MST was 12 months for GBM patients treated with a local field covering tumor plus less than 2 cm margin, 12 months for those treated with a generous field (2 cm or more margin), and 13 months for those treated to whole brain. Also for AA, whole brain radiation did not prolong survival. Initial relapse of GBM and AA developed within the irradiated volume in 86% of the cases treated with a generous field. Whole brain radiation seemed useless for the treatment of malignant gliomas. Survival time appeared to be dose-dependent; MST was 10, 13, and 16 months for GBM patients who received 45-57, 57-63, and 63-72 Gy, respectively. Extensive surgical resection was associated with a better prognosis in GBM. AA patients 60 years old or older had a poorer prognosis than younger patients, but age was not a significant prognostic factor for GBM. Chemotherapy appeared to prolong survival slightly without improving long-term survival.(ABSTRACT TRUNCATED AT 250 WORDS)

The ocular effects of intracarotid bromodeoxyuridine and radiation therapy in the treatment of malignant glioma

Since July 1985, 23 patients have been entered into a phase I/II clinical trial using intraarterial 5-bromodeoxyuridine (BUdR) (400-600 mg/m2 daily for 8.5 weeks) and focal external beam radiotherapy (59.4 Gy at 1.8 Gy daily in 6.5 weeks) in the treatment of malignant gliomas (Kernohan grades 3 and 4). The side effects in all patients have included varying degrees of anorexia, fatigue, ipsilateral forehead dermatitis, blepharitis, and conjunctivitis. Mucopurulent conjunctivitis and exposure keratitis developed in several patients and spontaneous corneal perforation developed in one. Eyes from two individuals examined at autopsy showed significant changes. Animal studies that predated clinical trials using rhesus monkeys did not predict the ophthalmologic complications seen in human subjects.

Random sampling by glitter drop method

In the course of a study on glioblastoma tissue and cultured cell lines, a glitter drop technique for random sampling was introduced (1). Here, we report the initial description of the details of this procedure and its application to cell counts from explant slides and to determining bromodeoxyuridine (BUdR) labeling indices (LI) in tissues. The method is without bias, easy to perform and takes only a short time. It compares well with three other generally used random sampling procedures.

Mucocutaneous complications of intraarterial 5-bromodeoxyuridine and radiation

5-Bromodeoxyuridine (BUDR), a halopyrimidine thymidine analogue, is incorporated into the DNA of dividing cells and causes photoradiosensitization. Twenty-five patients with malignant astrocytomas were treated with continuous intracarotid BUDR radiosensitization and radiotherapy for 8 1/2 weeks. Unique dose-limiting mucocutaneous complications were encountered. Ipsilateral facial dermatitis with epilation of eyebrows and eyelashes, ocular irritation, and bilateral nail dystrophy developed in all patients. Less common reactions included oral ulceration in six patients, body exanthem on the trunk in five, and atypical erythema multiforme major in one.

Radiobiology and clinical application of halogenated pyrimidine radiosensitizers

Halogenated pyrimidines (HP) represent a unique class of non-hypoxic cell radiosensitizers currently under clinical re-investigation. In order for halogenated pyrimidines to sensitize cells to radiation, they must be incorporated into cellular DNA. In the case of human tumors, which have in general rather long cell cycle times, this may require many days of continuous drug infusion to achieve adequate replacement of the DNA base thymidine with HP. In vitro studies support the relationship between the extent of radiosensitization and the percentage of thymidine replacement. Recent clinical studies evaluating the role of iododeoxyuridine (IdUrd) as a radiation sensitizer in large unresectable sarcomas have been extremely encouraging. To support and expand upon these positive clinical findings more information and research is needed regarding: (1) the mechanism of HP-induced radiosensitization; (2) the percentage of HP thymidine replacement in human tumors achievable and how it relates to treatment outcome; (3) the means of increasing HP incorporation in tumor and minimizing incorporation in normal tissues; (4) a better understanding of optimal timing between HP administration and radiation treatment; and (5) methods to evaluate which tumors are appropriate candidates for HP therapy. While presently limited to use in conventional high dose-rate X-ray therapy, laboratory studies suggest that HP might also be effective in low dose-rate brachytherapy and for selected high LET clinical beams. HPs probably will not be 'general' non-hypoxic cell radiosensitizers for all tumor types, but with appropriate tumor-type/anatomical site selection and refinement in their administration, HPs may prove beneficial in cancer treatment.

Mixing in the human carotid artery during carotid drug infusion studied with PET

The safety and efficacy of drug infusion into the carotid artery require adequate mixing of the infused solution with carotid blood. Using positron emission tomography (PET), we studied the mixing of solutions infused into the human carotid artery in seven patients by analyzing the distribution of [15O]H2O infused into the carotid artery and by vein. At four infusion rates ranging from 0.5 to 10 ml/min, the variability in distribution averaged 16.5-17.8% among the pixels in a large volume of interest, without dependence on the infusion rate. The overall correlation between [15O]H2O influx with arterial infusion and [15O]H2O influx with venous injection was 0.78-0.82 at the four infusion rates, with no trend toward higher correlations at the faster infusion rates. The distribution into the anterior, middle, and posterior cerebral artery territories differed from distribution throughout the entire carotid territory by an average of 6.2-9.6% at the four infusion rates, with no trend toward smaller differences at the faster infusion rates. Infusions performed into a vinyl tube simulating the carotid artery indicated that at 0.5 ml/min, the velocity of fluid exiting the catheter makes no apparent contribution to mixing. We conclude that with infusions at the carotid bifurcation, mixing in the human carotid artery is complete or nearly complete over a wide range of infusion rates. The mixing appears to result from the patterns of blood flow within the artery, and not from jet effects at the catheter tip.