Phase 1 clinical trial of recombinant interleukin-2: a comparison of bolus and continuous intravenous infusion

Continuous infusion interleukin-2 and intravenous famotidine in metastatic melanoma

Lymphokine-activated killer cell (LAK) cytotoxicity against tumor cells is induced by the use of high-dose infusional interleukin-2 (IL-2). LAK cytotoxicity against neoplastic cells may be augmented by famotidine. Twelve (12) patients have been treated with continuous infusion IL-2 (18 MIU/m2/24 hours) for 72 hours and famotidine 20 mg IVPB twice per day. Cycles were repeated every 3 weeks. These patients were of median age--67 years (range, 25-79), had a median performance status of 1 (range, 0-1), and had metastatic sites, including lung, lymph node, subcutaneous/soft tissue, and liver. The most common toxicities of this regimen were fever, rigors, nausea/emesis, hypophosphatemia, and hypomagnesemia. Three (3) partial responses have been seen (25% response rate). One (1) of these responders has undergone complete surgical resection and is disease-free at 15+ months. Four (4) patients are alive at a median of > 25 months. The median survival for all patients is 13 months. This combination of infusional IL-2 with famotidine is active in metastatic melanoma.

Pediatric Phase I Trials in Oncology: An Analysis of Study Conduct Efficiency

Purpose

To determine the efficacy and safety of pediatric phase I oncology trials in the era of dose-intensive chemotherapy and to analyze how efficiently these trials are conducted.

Methods

Phase I pediatric oncology trials published from 1990 to 2004 and their corresponding adult phase I trials were reviewed. Dose escalation schemes using fixed 30% dose increments were studied to theoretically determine whether trials could be completed utilizing fewer patients and dose levels.

Results

Sixty-nine pediatric phase I oncology trials enrolling 1,973 patients were identified. The pediatric maximum-tolerated dose (MTD) was strongly correlated with the adult MTD (r = 0.97). For three-fourths of the trials, the pediatric and adult MTD differed by no more than 30%, and for more than 85% of the trials, the pediatric MTD was less than or equal to 1.6 times the adult MTD. The median number of dose levels studied was four (range, two to 13). The overall objective response rate was 9.6%, the likelihood of experiencing a dose-limiting toxicity was 24%, and toxic death rate was 0.5%.

Conclusion

Despite the strong correlation between the adult and pediatric MTDs, more than four dose levels were studied in 40% of trials. There appeared to be little value in exploring dose levels greater than 1.6 times the adult MTD. Limiting pediatric phase I trials to a maximum of four doses levels would significantly shorten the timeline for study conduct without compromising safety.

Echinacea purpurea and melatonin augment natural-killer cells in leukemic mice and prolong life span

OBJECTIVE

We recently showed that daily dietary administration of Echinacea purpurea root extract to normal mice for as little as 1 week resulted in significant elevations of natural-killer (NK) cells (immune cells that are cytolytic to virus-containing cells and many tumor cells). Such boosting of this fundamental immune cell population suggests a prophylactic role for this herb in normal animals. Based on this evidence, our goal in the present work was to assess the role of dietary administration of this herbal extract to mice bearing leukemia, a type of tumor well known to be a target for NK cells.

DESIGN

A commercially available root extract of E. purpurea, which we have already shown to be highly effective in mice, was administered daily for 50 days from the onset of leukemia (day 0). Control leukemic mice received no extract. Other leukemic mice received the NK-enhancing neurohormone, melatonin, administered precisely as above. In all treatment and control categories, some mice were sampled at 9 days after tumor onset, others were sampled at 3 months, and still others were left to assess treatment effect on life span.

RESULTS

At 9 days (intermediate stage leukemia; death beginning by day 17-18), E. purpurea-treated mice had a 2.5-fold increase in the absolute numbers of NK cells in their spleens. By 3 months after leukemia onset, E. purpurea-treated mice still had 2-3 times the normal numbers of NK cells in their spleens. No leukemic, untreated (control) mice remained alive at 3 months, hence the comparison with normal animals. Moreover, at 3 months post-tumor onset, all the major hemopoietic and immune cell lineages in their bone marrow birth site, were recorded at normal numbers, in E. purpurea-consuming, leukemic mice. The survival advantage provided by administering these leukemic mice with E. purpurea was highly significant versus untreated, leukemic mice when analyzed by Kaplan-Meier survival statistics.

CONCLUSION

The present study has provided the first systematic analysis, under controlled laboratory conditions, of the effect(s) of the botanical, E. purpurea, in vivo, in leukemic hosts. The profoundly positive effects of this herb in disease abatement observed in this study suggest the therapeutic potential of E. purpurea, at least with respect to leukemia, if not other tumors as well.

Interleukin 2 maintains biologic stability and sterility over prolonged time

The FDA approved interleukin 2 (IL2) for clinical use in 1992 in a high-dose bolus intravenous infusion schedule. IL2 administered by continuous low- and intermediate-dose infusion can result in a variety of immunologic effects including the expansion of the Natural Killer (NK) cell pool and immune reconstitution in immune-deficient hosts. These immune modifications are essential for augmentation of both currently available and evolving immunotherapies. The manufacturer's data indicate stability of the IL2 for a period of 6 days. This time frame is not practical for prolonged infusional schemes necessitating frequent changes of drug depots. We tested the biologic stability and sterility of the commercially available recombinant IL2 preparation (aldesleukin; Proleukin, Chiron) under clinical conditions for up to 30 days. Our results confirm that IL2 retains its biologic activity and sterility under these conditions for prolonged periods. This information will simplify IL2 outpatient regimens, allowing for convenient intervals for drug depot renewal, leading to improved patient compliance and conserved health care expenditures.

IL-2 Plasmid Therapy of Murine Ovarian Carcinoma Inhibits the Growth of Tumor Ascites and Alters Its Cytokine Profile

We have evaluated whether i.p. murine ovarian tumors could be treated with an IL-2 plasmid DNA complexed with the cationic lipid, (±)-N-(2-hydroxyethyl)-N,N-dimethyl-2,3-bis(tetradecyloxy)-1-propanaminium bromide/dioleoylphosphatidylethanolamine (DMRIE/DOPE). Reporter gene studies were initially conducted in which mice bearing i.p. murine ovarian teratocarcinoma (MOT) were injected i.p. with reporter gene plasmid DNA (pDNA):DMRIE/DOPE. Histochemical analyses revealed that transfection occurred primarily in the tumor cells of the ascites, with only a minority of other ascitic cells or surrounding tissues transfected. IL-2 levels in the MOT ascites were determined after i.p. injection of either IL-2 pDNA:DMRIE/DOPE or recombinant IL-2 protein. IL-2 was detected in tumor ascites for up to 10 days after a single i.p. injection of IL-2 pDNA:DMRIE/DOPE, but was undetectable 24 h after a single i.p. injection of IL-2 protein. In an antitumor efficacy study, MOT tumor-bearing mice injected i.p. with IL-2 pDNA:DMRIE/DOPE on days 5, 8, and 11 after tumor cell implant had a significant inhibition of tumor ascites (p = 0.001) as well as a significant increase in survival (p = 0.008). A cytokine profile of the MOT tumor ascites revealed that mice treated with IL-2 pDNA:DMRIE/DOPE had an IL-2-specific increase in the levels of IFN-γ and GM-CSF. Taken together, these findings indicate that i.p. treatment of ovarian tumors with IL-2 pDNA:DMRIE/DOPE can lead to an increase in local IL-2 levels, a change in the cytokine profile of the tumor ascites, and a significant antitumor effect.

Strain differences in natural killer cell-mediated immunity among mice: a possible mechanism for the low natural killer cell activity of A/J mice

Natural killer (NK) cells are well established as fundamental elements in the early eradication of aberrant cells potentially leading to neoplasia. Moreover, it has also long been known that inbred strains of laboratory mice, as well as human individuals, demonstrate a wide range of NK cell-mediated immune response even to the same tumor. In the present study, various parameters which could lead ultimately to high, or low, NK cell-mediated functional activity have been assessed. Mice of the A/J strain demonstrate very low NK cell tumor-lytic activity and correspondingly high incidence of lymphoma. By contrast, C57B1/6 mice demonstrate relatively high NK cell activity and virtually never develop lymphomas. The results of this study have revealed that the absolute numbers of splenic NK cells were significantly lower in A/J vs C57B16/mice. Furthermore, the blood of A/J mice contained significantly fewer (30%) NK cells than did that of C57B1/6 mice. However, no significant difference between the 2 strains was found in the numbers of lymphocytic cells from NK cell-enriched fractions from the spleens, which possessed either the homing receptor MEL-14, or the integrin Mac-I, both essential surface molecules for transendothelial migration of lymphocytic cells from the circulation into organ parenchyma. Moreover, NK cells from both strains responded similarly to the NK cell stimulants, ATRA, indomethacin and interleukin-2. Finally, there was no significant difference between the 2 strains, in the numbers of lymphocytic cells in the bone marrow (including NK cells), which were radiolabelled with the DNA synthetic precursor, 3H-thymidine, indicative, thus, of equivalent levels of lymphocyte production by the bone marrow in the 2 strains. The observations collectively suggest that the low peripheral (spleen, blood) levels of NK cell-mediated functional activity found in the A/J strain of mouse at least, reflects either post-production, large-scale NK cell abortion/death, or a bone marrow-based microenvironmental deficiency which inhibits NK cells' exit from the bone marrow birth site.

TNF-alpha release from human peripheral blood mononuclear cells to predict the proinflammatory activity of cytokines and growth factors

Therapeutic applications of novel growth factors and cytokines can be limited due to the oftentimes severe inflammatory reactions that are encountered in patients following parenteral administration. These inflammatory responses, hallmarked by fever, headache, and chills are mediated by the release of inflammatory cytokines following administration of protein-based therapeutics. TNF-alpha appears to be one of the most potent and pleiotropic inflammatory cytokines released during the early stages of an inflammatory response. In the present study, we have explored the use of an in vitro assay system to determine whether the release of TNF-alpha from human PBMC could be correlated with documented inflammatory activity in humans. The cytokines tested included rhGM-CSF, rhIL-2, rhIL-3, and rhIFN-gamma, rhG-CSF, which has been shown to lack substantial proinflammatory activity in humans, was also tested. The potent macrophage activator and inflammatory agent, LPS, was used as a positive control. Results of this study demonstrate and confirm an association between the inflammatory activity of cytokines observed in patients and the stimulation of TNF-alpha release from PBMC in vitro. This assay system can be used as a predictive tool in determining the inflammatory potential of novel growth factors and cytokines.

A phase II trial of human recombinant interleukin-2 administered as a 4-day continuous infusion for children with refractory neuroblastoma, non-Hodgkin's lymphoma, sarcoma, renal cell carcinoma, and malignant melanoma. A Childrens Cancer Group study

BACKGROUND

Recombinant human Interleukin-2 (IL-2) has been effective at inducing measurable antitumor responses in adults with renal cell carcinoma and melanoma. It also is being tested as adjuvant therapy for patients with acute myeloid leukemia after autologous bone marrow transplantation.

METHODS

The authors tested IL-2 in a pediatric Phase II trial using a regimen that has antitumor effects in adults and was proven to be tolerated acceptably in a prior Phase I pediatric trial. Thirty-eight patients were entered into this study of whom 36 received IL-2 and were evaluable (20 with sarcoma, 9 with neuroblastoma, 5 with renal cell carcinoma, and 1 each with melanoma and lymphoma).

RESULTS

Interleukin-2 dose modifications were based on tolerance and toxicity, such that 46% of these patients received a 50% increase in IL-2 dose during the second week, and 81% of those receiving the elevated dose continued receiving this dose level during the third week of treatment. A single patient with renal cell carcinoma had a complete response that was maintained; the remaining 35 patients did not show objective evidence of tumor response sufficient to qualify as either a complete response or a partial response.

CONCLUSIONS

Absolute lymphocyte counts were indicative of the immunostimulatory effect of this IL-2 regimen as observed for adults, with a median 7.2-fold increase. Nevertheless, despite immune activation, a sufficient number of patients were evaluated, indicating that IL-2 does not have measurable antitumor effects in children with large refractory sarcomas or neuroblastomas, whereas one of five children with renal cell carcinoma had a complete response, consistent with the 10-20% response rate observed in adults.

Clinical and immunological effects of human recombinant interleukin-2 given by repetitive weekly infusion to normal dogs

Four normal adult dogs received two consecutive weekly cycles of human recombinant interleukin-2 (IL-2) by continuous infusion for 4 days/week. The dose of IL-2 given to each dog was 3 x 10(6) units m-2 day-1. Toxicities consisted of mild vomiting, diarrhea, and lethargy to varying degrees in all the dogs. These side-effects were reversed when the treatment was discontinued. Fever, tachypnea, and weight gain were not seen. A marked lymphocytosis and eosinophilia developed in all dogs after completion of each course of IL-2 (resulting in a more than sevenfold increase in each cell type) and persisted for more than 1 month in some. Fresh peripheral blood lymphocytes (PBL) obtained during this lymphocytosis mediated enhanced in vitro lysis of a natural-killer-cell-sensitive canine tumor cell line (CTAC). The in vitro proliferative responses of these same PBL to IL-2 could be detected earlier, progressed faster, and involved more cells than PBL tested prior to IL-2 infusion. Thus, a relatively well-tolerated regime of IL-2 in dogs can induce dramatic increases in lymphocyte numbers and activation, which is associated with augmentation of their in vitro antitumor reactivity. The clinical effectiveness of this immunotherapeutic approach remains to be tested in tumor-bearing dogs where it could serve as a relevant large-animal model for immunotherapy of cancer with IL-2.

Decline in natural killer cell-mediated immunosurveillance in aging mice--a consequence of reduced cell production and tumor binding capacity

The mechanism(s) responsible for the functional deficiencies in aging mice of natural killer (NK) cells, the lymphocytes active in early neoplasia development, are unknown. The aim of the present study was to examine the effect of indomethacin, a prostaglandin synthetase inhibitor, and recombinant interleukin-2 (rIL-2), an NK cell stimulant, on the NK cell numbers and function of aging mice. This combination is highly effective in inducing proliferation and activation of the NK cells in young adult mice. Ten- to 13-month-old DBA/2 mice received either indomethacin daily for 9 days in their drinking water, or rIL-2 twice a day for 4 days, or both agents combined. Untreated aging mice had 50% fewer splenic NK cells than did young adult (5-8 weeks) DBA/2 mice, although bone marrow contained NK cell numbers similar to that of young adult mice. Neither indomethacin alone, rIL-2 alone nor the combination of both could induce an increase in NK cell numbers or function in aging mice. Radioautography combined with immunoperoxidase labelling techniques revealed that the production of new NK cells from the bone marrow of aging mice was significantly reduced relative to that of young adult mice. This was reflected in low numbers of newly formed NK cells accumulating in the spleens of aging mice. The target binding capacity, a necessary precytolytic event, of aging mouse NK cells was also reduced compared to that of young adult mice. The results suggest that the functional deficiencies of aging mouse NK cells are a consequence of multiple factors including an absolute loss of NK cells resulting from a reduced production of such cells in the bone marrow and a decreased capacity of NK cells to bind their tumor targets.

A phase I study of prolonged continuous infusion of low dose recombinant interleukin-2 in melanoma and renal cell cancer. Part II: Immunological aspects

Previously we described the clinical aspects of a phase I study of prolonged continuous infusion of low-dose recombinant interleukin-2 (rIL-2). In the present paper we report several immunological effects in 13 patients with melanoma and renal cell cancer treated on an out-patient basis with rIL-2 for uninterrupted periods ranging from 5 to 18 weeks. Groups of three patients were treated at following dose levels 0.18, 0.6, 1.8 or 6 x 10(6) IU m-2 24 h-1 and one patient was treated with 3 x 10(6) IU m-2 24 h-1. Prolonged rIL-2 treatment resulted in a dose-dependent and sustained increase in the percentage and absolute number of (CD56+, CD8dim) natural killer cells. Within this population a preferential increase in the CD56bright cells with low expression of CD16 was observed. The CD27 antigen was also upregulated in the CD56bright CD16dim population. This increase of NK cells was accompanied by an enhancement of the cytotoxic capacity of the peripheral lymphocytes. No consistent signs of T cell activation or expansion were noted.

Immunomodulation during prolonged treatment with combined interleukin-2 and interferon-alpha in patients with advanced malignancy

Treatment with combined IL-2 and alpha-IFN has resulted in synergistic antitumour efficacy in animal studies. The mechanisms responsible for this synergy remain unclear. In this study, several immune parameters which might be involved in mediating antitumour activity have been monitored serially in 15 patients with advanced malignant melanoma or renal cell cancer during treatment with concurrent IL-2 and alpha-IFN. Both drugs were given subcutaneously in low to moderate (outpatient) dosages but for a prolonged duration. This treatment resulted in remarkable immunomodulation. In vivo induction of cytotoxicity against K562 and Daudi target cells was consistently seen, and percentages of peripheral blood cells expressing CD 25 (IL-2 receptor) and CD 56 (Leu-19) increased. In vitro proliferation of lymphocytes in response to IL-2 was enhanced during the treatment periods, whereas spontaneous proliferation was inhibited. Moreover, correlations between immune parameters and subsequent clinical responses were present in the early phase of the study. Cytotoxicity levels generated in vivo as well as the percentage of CD 56+ lymphocytes were higher in patients who responded to treatment than in non-responders. In contrast, responders had lower levels of CD 25+ cells. These findings indicate that it might be possible to select patients who are likely to benefit from prolonged immunotherapy.

A phase I study of prolonged continuous infusion of low dose recombinant interleukin-2 in melanoma and renal cell cancer. Part I: Clinical aspects

The optimal schedule for recombinant interleukin-2 (rIL-2) administration is unclear. Because the clinical and immunological effects of prolonged continuous exposure to rIL-2 are unknown, we have conducted a phase I study to assess the toxicity and feasibility of continuous low dose infusion of rIL-2 (EuroCetus) using central venous access with a portable infusion device on an out-patient basis. Twenty-two patients entered the study, 13 with melanoma and nine with renal cell cancer, age range 26-66 years (median 51), performance status less than or equal to 1. They were treated with one of the following doses per m2 per 24 h: 0.18 x 10(6) IU, 0.6 x 10(6) IU, 1.8 x 10(6) IU, 3 x 10(6) IU, 6 x 10(6) IU and 9 x 10(6) IU. Toxicity was evaluable in 20 patients receiving greater than or equal to 3 weeks treatment duration or in whom treatment was discontinued prematurely because of toxicity. Constitutional symptoms consisting of fatigue, malaise and fever up to 40 degrees C without significant organ dysfunction occurred with doses greater than or equal to 1.8 x 10(6) IU m-2. The maximum tolerated dose was 6 x 10(6) IU m-2 24 h-1. In all patients toxicity reached a peak at 3 weeks and resolved thereafter despite continued rIL-2 treatment. Peripheral blood eosinophilia (up to 66% of white blood cell count) followed the same pattern. An infection of the central venous access occurred in 55% of the patients but this was mostly asymptomatic. Thirteen patients were treated greater than or equal to 6 weeks and were evaluable for tumour response. A partial remission occurred in a patient with melanoma with a dose of 1.8 x 10(6) IU rIL-2 m-2 24 h-1.

Clinical applications of interleukin-2

Interleukin-2 (IL-2) is a cytokine with potent immunomodulating properties which has shown considerable antitumour activity in preclinical models. In clinical trials, the effects of IL-2 given by various routes and schedules have been investigated. IL-2 has been administered either as single drug or in combination with other cytokines and immunomodulating agents, chemo therapeutic agents, or reinfusions of ex vivo activated autologous cytotoxic effector cells. The results of published clinical studies with IL-2 based immunotherapy are reviewed in this paper.

Population dynamics of natural killer cells in the spleen and bone marrow of normal and leukemic mice during in vivo exposure to interleukin-2

By quantitative and functional methods, changes were assessed in NK(ASGM-1+) cell numbers and NK cell-mediated lytic function of the spleen and bone marrow of mice bearing a tumor of hemopoietic origin (FLV-induced erythroleukemia) for 9 days +/- simultaneous administration of indomethacin (10 micrograms/ml drinking water) +/- rIL-2 (3x/day, 12 x 10(3) Units/injection) during the last 4 days of tumor-bearing. Recombinant IL-2 alone during the last 4 days of tumor-bearing increased both the NK(ASGM-1+) cell numbers (p less than 0.001) and the functional activity (24-fold) of the spleen. In the bone marrow, however, no change in the numbers of NK(ASGM-1+) cells was observed relative to untreated tumor-bearing mice, but the NK cell-mediated lytic activity of that organ was augmented 30-fold. The continuous presence of indomethacin from the onset of tumor-bearing prior to rIL-2 treatment during the last 4 days of tumor-bearing, further boosted both the already high, rIL-2 driven numbers of NK(ASGM-1+) cells in the spleen (p less than 0.01), as well as splenic NK cell lytic function (2-fold). In the bone marrow, continuous presence of indomethacin prior to and during the terminal 4 days of co-administration with rIL-2 increased 3-fold the numbers of NK(ASGM-1+) cells relative to that of the bone marrow of tumor-bearing mice given rIL-2 alone, and resulted in lytic activity of that organ which was 140% of that of the rIL-2 treated, tumor-bearing mice. The results indicate that under the combined influence of indomethacin and rIL-2, the production of NK(ASGM-1+) cells was augmented in the bone marrow of tumor-bearing mice, export of immature NK(ASGM-1+) cells from the bone marrow was increased, and import of immature NK(ASGM-1+) cells by the spleen was increased. The increased NK(ASGM-1+) cell numbers in each organ was reflected in increased lytic function.

Thrombocytopenia during immunotherapy with interleukin-2 by constant infusion

PURPOSE

To elucidate some of the possible mechanisms that lead to interleukin-2 (IL2)-induced thrombocytopenia.

PATIENTS AND METHODS

We evaluated retrospectively the effects of immunotherapy with IL2 in 76 patients with disseminated cancer. The lymphokine was administered by constant infusion, daily for 6 days a week for 4 consecutive weeks.

RESULTS

A significant decrease in platelet counts was seen after the first 6 days of therapy in all but two patients: 14 patients experienced grade 2 or 3 toxicity, 21 had grade 1 toxicity, and although the decrease in platelet counts could not be graded as toxicity in the remaining 41 patients, there was an average decrease of 32% from baseline platelet counts in 39 (p less than 0.0001). Thrombocytopenia appeared to be secondary to peripheral platelet destruction, since bone marrow biopsy specimens obtained during thrombocytopenia showed hyperplastic megakaryocytopoiesis. IL2 is inactivated by tubular resorption, and severity of thrombocytopenia was strongly correlated with IL2-induced renal dysfunction (p = 0.0004). Additionally, both renal dysfunction and thrombocytopenia were related to total dose of IL2 and were more pronounced in patients with worse baseline renal function and lower baseline platelet counts. The incidence of thrombocytopenia increased with subsequent IL2 therapy: life-threatening thrombocytopenia (less than 25,000/microL) was seen in nine of 57 patients, five of whom required transfusional platelet support.

CONCLUSION

On the basis of preliminary observations, we hypothesize that thrombocytopenia induced by IL2 is caused by accelerated clearance of platelets by the reticuloendothelial system.

Biologic response modifiers: the future of cancer therapy?

The major shift today has been away from nonspecific compounds acting on immune mechanisms to using biologics which have specific, defined roles in acting on the immune response. The field of biologic response modification is progressing very rapidly. New peptides are being identified, as are receptors for these peptides, autocrines, lymphokines, cytokines, growth factors, differentiation factors, hormones, and so on-all of which will control body function, cell populations, and cell to cell interactions. This rapidly advancing area of research in cancer biology and cancer therapy may hold the key to the future of successful therapy.

Addition of interleukin-2 in vitro augments detection of lymphokine-activated killer activity generated in vivo

The in vivo administration of repetitive weekly cycles of interleukin-2 (IL-2) to patients with cancer enhances the ability of freshly obtained peripheral blood lymphocytes (PBL) to lyse both the natural-killer(NK)-susceptible K562 and the NK-resistant Daudi targets. Lysis of both targets is significantly augmented by inclusion of IL-2 in the medium during the cytotoxicity assay. This boost is much greater for cells obtained following the in vivo IL-2 therapy than for cells obtained prior to the initiation of therapy or for cells from healthy control donors. In addition to direct lytic activity, the PBL obtained following in vivo IL-2 show a rapid increase in lymphokine-activated killer (LAK) activity with more prolonged in vitro IL-2 exposure, indicating that LAK effectors primed in vivo respond with "secondary-like" kinetics to subsequent IL-2 in vitro. Lymphocytes from healthy control individuals, cultured in IL-2 under conditions attempting to simulate the in vivo IL-2 exposure, function similarly to PBL obtained from patients following IL-2, in that low-level LAK activity was significantly boosted by inclusion of IL-2 during the cytotoxic assay and the cells also responded with secondary-like kinetics to subsequent IL-2 in vitro. The augmentation of the LAK effect was also dependent on the dose of IL-2 added during the 4-h 51Cr-release cytotoxicity assay, with higher doses of IL-2 having a more pronounced effect. While continuous infusion of IL-2 induces a greater cytotoxic potential per milliliter of blood obtained from patients, the peak serum IL-2 levels attained are greater with bolus IL-2 infusions. These pharmacokinetic results, together with the IL-2 dose dependence of LAK activity generated in vivo shown in this report, suggest that a combination of treatment with bolus IL-2 infusions superimposed on continuous IL-2 infusion may transiently expose IL-2 dependent LAK cells, activated in vivo, to higher concentrations of IL-2, facilitating their in vivo cytotoxic potential.