HYPOXIC CELL SENSITIZERS

Dr Bharti Devnani

Moderator:-Dr Ritu Bhutani

What is the Hypoxic cell sensitizer? What are the characteristics of an ideal Radio sensitizer? Enumerate all Radio sensitizers.

GOAL OF RADIATION THERAPY

CONCEPTS

Two concepts fundamental to understanding the

rationale for modification of radiation response

1. Therapeutic ratio – defined as NTT/TLD

Both of these parameters have sigmoid dose response curves

As the separation between these curves increases, the

likelihood increases that treatment will be effective & not

cause an unacceptable level of morbidity

2. Efficacy & toxicity profile of the modifier – directly affect

TR

A radiosensitizing agent that exacerbates toxicity to same

extent that it improves efficacy – TR unchanged or worsened

RADIOSENSITISERS

Physical and Chemical (pharmacological) agents that increase

the lethal effects of radiation when administered in

conjunction to radiotherapy.

TYPES OF SENSITISERS

Non hypoxic cell sensitisers(Halogenated pyrimidines )

Differential effect is based on the premise that tumor cells cycle faster and therefore incorporate more of the drug than the surrounding normal tissues.

Hypoxic-cell sensitizers

Increase the radiosensitivity of cells deficient in molecular oxygen(tumors) but have no effect on normally aerated cells.

Oxygen fixes the damage by free radical to DNA. (O2 fixation hypothesis)

Why o2 is required?

CHARACTERISTICS OF AN IDEAL HYPOXIC CELL SENSITIZER

1. Selectively sensitize hypoxic cells at concentration that would

result in acceptable normal tissue toxicity (differential effect)

2. Chemically stable & not subject to rapid metabolic break down

3. Highly soluble in water or lipids & must be capable of diffusing

a considerable distance through a nonvascularized cell mass

to reach the hypoxic cell

4. It should be effective at relatively low daily dose /# used in

conventional fractionated radiotherapy

METHODS TO SENSITIZE HYPOXIC CELLS

Physical Chemical

METHODS TO SENSITIZE OR ELIMINATE HYPOXIC CELLS

1. Physical

Overcoming hypoxia by eliminating it with treatment that increases

delivery of oxygen to tumor i.e. increases the oxygen carrying

capacity of blood and increasing the tumor blood flow

a) Hyperbaric oxygen

b) Carbogen with or without nicotinamide

c) ARCON

d) Hyperthermia

METHODS TO SENSITIZE OR ELIMINATE HYPOXIC CELLS2. Chemical

a) Modifiers of Hb

b) Hypoxic cell sensitizers

c) Hypoxic cytotoxins or bioreductive dugs - Pharmacological

targeting of hypoxic cells – cytotoxic to hypoxic tumor cells

d) Biologic modifiers

e) Chemotherapeutic drugs

Physical radiosensitizers

HYPERBARIC OXYGEN

An increase in barometric pressure of the gas breathed by the patient during radiotherapy is termed ‘hyperbaric oxygen (HBO) therapy’.

Pioneered by Churchill-Davidson in 1968 at St. Thomas' Hospital in London.

Patients were sealed in chambers filled with pure oxygen raised to a pressure of 3 atm.

HYPERBARIC OXYGEN

Problems

Feeling of claustrophobia

Unconventional hypofractionated schemes

Increase in late normal tissue damage(damage

to laryngeal cartilage in studies)

Risk of fire

Cumbersome Side effects - damage to the ears, sinuses and

lungs from the effects of pressure, temporary worsening of myopia, acute central nervous system oxygen toxicity (seizures)Discarded due to introduction of better chemical radiosensitisers that would achieve same end by simpler

means

Clinical trial of HBO

largest of the multicentre clinical trials of HBO is performed by the British Medical Research Council.

Results:-Benefit observed in (both LRC & OS) Advanced head and neck cancer Uterine cervix cancer

Benefit was not observed in bladder cancer;Improvement in LRC

(6.6%) & survival

CARBOGEN

Pure oxygen if breathed – vasoconstriction - closing

down of some blood vessels – defeats the object

Carbogen – 95% O2 +5% CO2

Rationale – addition of CO2 to gas breathing mixture -

shift the oxyHb association curve to right – facilitate

unloading of oxygen into most hypoxic tissues

Simple attempt to overcome chronic hypoxia

Can be given under normobaric condition. Failed to show significant therapeutic gain (Horsman et al., 2007).

Diffusion limited chronic hypoxiaHypoxia Perfusion limited acute hypoxia

NICOTINAMIDE(B3)

Prevents the transient fluctuations in tumour blood flow that lead to the development of acute hypoxia (Horsman et al.,1990).

Benefit has been seen when combined with Hyperthermia, Perfluorochemical emulsions, Pentoxifylline and High oxygen-content gas breathing (Horsman,1995).

ARCON

Accelerated – to overcome proliferation

Hyperfractionated – to spare late responding normal

tissues

Carbogen breathing – to overcome chronic hypoxia

Nicotinamide – to overcome acute hypoxia

PHASE III TRIAL

Laryngeal carcinoma(Netherland)

Increase in regional control rate 93% v/s 86%(p=.04) With equal toxicity

Chemical sensitizers

Modifiers of hemoglobin

BLOOD TRANSFUSION

Anemia – powerful adverse prognostic factor in pts of Ca

Cervix, H& N cancers & lung cancer

Investigated in no. of studies

1st clinical investigation – in advanced cervical cancer

Transfusion to pts with low Hb levels - ↑ed oxygen tension

within tumor

Transfusion to Hb level of 11g/dl or higher – improved survival

Not been supported by data from controlled randomized trials

H & N Cancer pts – 2 phase II trials from DAHANCA study

group – failed to demonstrate any benefit

ERYTHROPOETIN

Low Hb concentration ↓es radiation response of tumors

Two studies conducted in H & N cancers failed to show any benefit

In one of the studies – pts who received erythropoetin showed

significantly poor outcome than those who did not

? Erythropoietin may stimulate tumor growth

Schedules –

Thrice weekly – 150 U/kg s.c

Weekly - 40000 U s.c.

PERFLUOROCARBONS

Artificial blood substances

Small particles capable of carrying more oxygen or

manipulating the oxygen unloading capacity of blood

Potential usefulness uncertain

HYPOXIC CELL SENSITISERS

Nine different drugs have reached clinical evaluation

Misonidazole, Metronidazole, Benznidazole, Desmethyl-misonidazole Etanidazole, Pimonidazole Nimorazole, Ornidazole Rsu1069

Hypoxic Cell Radiosensitizers

The first candidate to satisfy these criteriawas Misonidazole

Hypoxic Cell Radiosensitizers

METRONIDAZOLE

1st generation 5-nitroimidazole

Sensitizer Enhancement ratio - 1.2

Formulations - 500 mg tablets or 500mg /100 ml solution

Half life – 9.8 hrs

Total cumulative dose not to exceed 54 gm/m2

Multiple doses 6gm/m2 3 times/wk for 3- 4week

Optimal time for administration - 4 hour before

radiation

Dose limiting toxicity –

Gastrointestinal

Sensory peripheral neuropathy

MISONIDAZOLE

2nd generation 2- nitroimidazole

Higher electron affinity

Sensitizer Enhancement ratio – 1.4 with multiple dose of 2 gm/m2 1.15 with 0.5mg/m2

Formulations 500 and 100 mg tablets and capsules

once or twice/wk for 5-6 wks

Total cumulative dose not to exceed 12 gm/m2

Optimal time for administration -- 4 hour before radiation

Dose limiting toxicity- gastrointestinal Sensory peripheral neuropathy that progress to

central nervous system toxicity

DAHANCA 2

DAHANCA 2, showed a highly significant improvement in thestratification subgroup of pharynx tumors, but not inthe prognostically better glottic carcinomas

ETANIDAZOLE (SR2508)

3rd generation, analog of Misonidazole

SER- 2.5-3 with dose of 12 g/m2

Arthralgia seen more often with 48 hr continuous infusion

1000mg/19.4 ml saline solution

Total dose - 40.8 g/m2 at 1.7-2g/m2 3 times/wk for 6 wks

30 min before radiationLesser neurotoxic due to

Shorter half life

Lower lipid solubility(less rapidly taken by the neural

tissue)No significant benefit was observed in two large head and neck cancer trials, one in the USAand the other in Europe.

ETANIDAZOLE

RTOG phase III study with Etanidazole in head and neck tumors

n- 521 patients

Conventionally fractionated RT RT

with Etanidazole 2mg/m2 with out Etanidazole

three times wk

No grade III or IV central nervous system or peripheral neuropathy was observed.

The 2-year actuarial local tumor control was 40% in each arm, and the survival was 41% and 43%, respectively, in the irradiation alone and the irradiation plus etanidazole arms

No overall benefit when Etanidazole

added to conventional radiotherapy

PIMONIDAZOLE

4- nitroimidazole

More potent than Misonidazole Uncharged at acid pH, thus promoting its accumulation in

ischaemic regions of tumours.

Several – fold ↑ in tumor concentration

Maximum tolerated dose – 750 mg/m2

Dose limiting toxicity – CNS manifesting as disorientation & malaise A pimonidazole trial was started in uterine cervix, but was stopped

when it became evident that those patients who received pimonidazole showed a poorer response.

NIMORAZOLE

A 5-nitroimidazole of same structural class as metronidazole

Administered in form of gelatin-coated capsules containing 500

mg active drug

Given orally 90 min prior to irradiation.

Daily dose 1200 mg/m2 body surface

Total dose should not exceed 40g/m2 or 75 g in total.

Less effective radio sensitizer then Misonidazole or Etanidazole

Less toxic, no cumulative neuropathy

Large dose can be given

dose-limiting toxicity is nausea and vomiting

NIMORAZOLE

NIMORAZOLE – DAHANCA 5

Significant improvement in terms of LRC & OS

Nimorazole significantly improves the effect of radiotherapeutic management of supraglottic and pharynx tumors and can be given without major

side-effects

As a consequence, nimorazole has now become part of the standard treatment schedule for head and neck tumours in Denmark.

DEVELOPMENT OF NITROIMIDAZOLES

Metronidazole

Misonidazole more active, toxic, benefit in subgroup

Etanidazole less toxic, not active

Nimorazole less active, much less toxic, benefit in H& N cancer

Summary of nitroimidazoles trials

NEWER NITROIMIDAZOLES

Doranidazole promising preliminary results were obtained

in a phase III study with intraoperative radiotherapy in advanced pancreatic cancer

Sanazol which in an International Atomic Energy

Agency multicentre randomized trial (Dobrowsky et al., 2007) in cervical cancer was found to significantly increase local control and survival following radical radiotherapy

OVERGAARD META-ANALYSIS 10,779 patients 83 RCT Hyperbaric oxygen (HBO) (28 trials), Hypoxic radiosensitizers (52 trials), Oxygen or carbogen breathing (3 trials), Blood transfusion (1 trial).

The tumor sites were Bladder (16 trials), Uterine cervix (15 trials), Central nervous system (13 trials), Head and neck (24 trials), Lung" (11 trials), Esophagus (2 trials), Mixed (2trials).

RESULTS OF META-ANALYSIS

Statistically significant benefit in LRC(4.7%) & OS (2.7%)

Locoregional tumor control was stastically significant in H&N (p=0.0002)

Hypoxic cytotoxins

HYPOXIC CYTOTOXINS BIOREDUCTIVE DRUGS

Elimination of radioresistant hypoxic cells by selectively killing them.

These compounds undergo intracellular reduction to form active cytotoxic species, primarily under low oxygen tensions.

Maximum cytotoxicity to cells at maximum distance from

tumor blood vessels

Overcome major cause of resistance of solid tumors –

inadequate oxygenation & drug delivery to tumor cells distant

to blood vessel

HYPOXIC CYTOTOXINS

Quinone antibiotics

• MMC• EO9• porfirom

ycin

Nitroaromatic compounds

• Misonidazole

• (Rb-6145)

• Nlcq-1, Cb1954, Sn23862

• Pr-104

QUINONE ANTIBIOTIC - MITOMYCIN C

Prototype bioreductive drug

Used as chemotherapy agent for many years

Cytotoxic to relative radio resistant hypoxic cells

But the differential cytotoxicity between hypoxic and

oxygenated cells , however is small

Acts as an alkylating agent after intracellular activation &

inhibits DNA – DNA cross linking, DNA depolymerization

Dose limiting toxicity – cumulative myelosuppression

Mitomycin C plays an important role in conjunction with

radiotherapy and 5FU, the definitive, chemoradiation

squamous cell carcinoma of the anus

PORFIROMYCIN

A mitomycin C derivative

Provides greater differential cytotoxicity between hypoxic and

oxygenated cells in vitro

Phase III study

Compared patients treated with conventionally fractionated

radiation plus mitomycin C versus radiation plus porfiromycin

The median follow-up - >6 years. Hematologic and non-

hematologic toxicity was equivalent in the two treatment arms

Mitomycin C was superior to porfiromycin with respect to 5-

year local relapse-free survival (91.6% vs. 72.7%; p = 0.01)

Local-regional relapse-free survival (82% vs. 65.3%; p =

0.05)

Disease-free survival (72.8% vs. 52.9%; p = 0.03)

There were no significant differences between the two

arms with respect to overall survival (49% vs. 54%) or

distant metastasis-free rate (80% vs. 76%)

Their data supported the continuing use of mitomycin C as

an adjunct to radiation therapy in advanced head and neck

cancer and will become the control arm for future studies

PORFIROMYCIN…

TIRAPAZEMINE (SR 4233) Highly selective toxicity against hypoxic cells both in vivo and

vitro

MOA- Drug is reduced by intracellular reductases to form highly

reactive radical - produces both double & single strand breaks in

DNA

Analysis of DNA and chromosomal breaks after hypoxic exposure

to Tirapazemine suggests that DNA double-strand breaks are the

primary lesion causing cell death

Efficacy depends on no. of effective doses that can be

administered during course of RT & presence of hypoxic tumor

cells

S/E – nausea & muscle cramping

TIRAPAZEMINE

Hypoxic/cytotoxicity ratio –

ratio of drug concentration under

aerated and hypoxic condition

required to produce same cell

survival

Unlike the oxygen-mimetic

sensitizers, tirapazamine-

mediated therapeutic

enhancement occurs both when

the drug is given before or after

irradiation.

Tirapazamine can also

enhance the cytotoxicity of

cisplatin

N= 121 STAGE III/IV SCC OF THE HEAD AND NECK RANDOMIZED TO RECEIVE DEFINITIVE RADIOTHERAPY (70 GY IN 7 WEEKS)

Tirapazamine On day 2 of weeks 1, 4, and 7,290 mg/m2 was administered for 2 hours, followed 1 hour later by cisplatin 75 mg/m2 for 1 hr

followed immediately by radiotherapy

In addition, tirapazamine 160 mg/m2 was given before radiation three times/week in weeks 2 and 3

Cisplatin 50 mg/m2 was given before radiotherapy on day 1 of weeks 6 and 7 of radiotherapy

and

Fluorouracil 360 mg/m2/d was given by continuous infusion from day 1 - 5 (120-hour infusion) of weeks 6 and 7 of radiotherapy

Arm 2. n-58Arm 1,n-62

Three-year failure-free survival rates were 55% with TPZ/CIS

and 44% with chemo RT( p .16)

Three-year locoregional failure-free rates were 84% in

the TPZ/CIS arm and 66% in the chemo RT arm (p .069)

Toxicity

More febrile neutropenia and grade 3 or 4 late mucous

membrane toxicity were observed with TPZ/CIS

Compliance with protocol treatment was satisfactory on both

arms

Markers of hypoxic cells

MARKERS OF HYPOXIC CELLS

Radioactive labeled nitroimidazoles

Bioreduction Deposition

of radionuclide

Quickly excretedWithout

breaking down

Hypoxia Aerobic tissue

RADIOACTIVE LABELED NITROIMIDAZOLES

Nitroimidazole can be labeled with I 123

Hypoxic region of tumour can be visualized with single

photon emission computed tomography

Now, tumor Hypoxia can be detected by [18F]-Misonidazole

Positron Emission( FMISO-PET) in Patients With Advanced

Head and Neck Cancer imaging

Noninvasive procedure that can be used as predictive assay

in individual patients

The availability of methods to detect significant areas of

hypoxia can allow selection of patients who may benefit from

method of overcoming hypoxia

CONCLUSION 1896 -First radiotherapy treatment. 1909 -First clinical observation by Gollwald Schwarz

showing Reduced blood flow caused radioresistance. 1953- First experimental observation of potential

importance of hypoxia in radiotherapy. 1955 -First observation of hypoxia in human tumors. 1955 -First hyperbaric treatment. 1968- Results from first randomized trial. 1976 -First randomized study with hypoxie

radiosensitizer. 19,95- More than 10000 patients in 83 randomized

trials. Metaanalysis shows highly significant survival and local control benefit.

2013- Still no impact on general dinical practice.

NOVEL DRUGS

5 promising redox modulators are in development.

Tirapazamine AQ4N RSR13 facilitates delivery of oxygen to

tumor cells, thereby rendering them more sensitive to radiation.

Motexafin gadolinium, with a porphyrin-like structure, selectively accumulates in tumor cells and thereby enhances radiation-induced DNA damage.

HIF-1 inhibitors target a transcription factor that regulates hypoxia related events and cell survival.

HIF-1 TARGETING AGENTS

Soluble guanylyl cycle activators (e.g., YC-1) (84, 85),

HSP90 inhibitors (e.g., geldanamycin radicicola) (80, 81, 86),

PI3K inhibitors (e.g., wortmannin, LY294002) mTOR inhibitors(e.g., Rapamycin, CCI-779) Microtubule modifiers (e.g., 2-

methyloxyestradiol, vincristine,Taxol) topoisomerase I inhibitors