Background and Principle

AkaLumine hydrochloride is a near-infrared luciferin analogue developed to overcome the depth limitations of conventional bioluminescence imaging. Standard D-luciferin reacts with firefly luciferase (FLuc) to produce photons at ~560 nm, a wavelength strongly absorbed by hemoglobin and other tissue chromophores. AkaLumine was designed to pair with Akaluc, an engineered luciferase, to generate bioluminescent emission in the 650–700 nm range. At these longer wavelengths, photon penetration through mammalian tissue is substantially greater, and tissue autofluorescence is reduced, enabling detection of reporter signals from deeper anatomical compartments such as the thorax and abdomen in rodent models.

The Akaluc/AkaLumine system is particularly advantageous for tracking small numbers of cells (e.g., circulating tumor cells, stem cells, immune cells) in living animals, for longitudinal tumor studies, and for any application where high sensitivity and spatial resolution at depth are required. Because AkaLumine is a hydrochloride salt, it offers improved aqueous solubility compared to some other luciferin analogues, facilitating preparation of stable dosing solutions.

Materials and Equipment

Reagents:

• AkaLumine hydrochloride (Aladdin Cat. No. T487470)
• Dimethyl sulfoxide (DMSO), anhydrous, for stock preparation
• Phosphate-buffered saline (PBS), pH 7.4, sterile
• Cell culture media appropriate for your cell line
• Akaluc-expressing cell line or animal model

Equipment:

• Analytical balance and calibrated pipettes
• Microcentrifuge tubes (1.5 mL, low-binding recommended)
• Biosafety cabinet (for cell work)
• Luminometer or microplate reader with luminescence module (for in vitro assays)
• In vivo optical imaging system capable of detecting NIR emission (650–750 nm bandpass filter recommended)
• Isoflurane anesthesia system (for rodent in vivo imaging)
• Insulin syringes (29–31 G) for subcutaneous or intraperitoneal injection
• −20 °C and −80 °C freezers for reagent storage

Step-by-Step Protocol

Part A: Preparation of AkaLumine Stock Solution

1. Allow the AkaLumine hydrochloride vial to equilibrate to room temperature inside a desiccator before opening to prevent condensation.
2. Weigh the required amount of AkaLumine hydrochloride using an analytical balance.
3. Dissolve in anhydrous DMSO to prepare a concentrated stock (e.g., 50–100 mM). Vortex gently and ensure complete dissolution. Note: Avoid prolonged exposure to light; wrap tube in foil.
4. Aliquot into single-use volumes to minimize freeze-thaw cycles. Store aliquots at −20 °C (short-term, ≤1 month) or −80 °C (long-term). Do not store in frost-free freezers.
5. For aqueous working solutions, dilute the DMSO stock into sterile PBS (pH 7.4) immediately before use. Keep the final DMSO concentration ≤1% (v/v) for cell-based assays or ≤5% (v/v) for in vivo injection (verify tolerability for your model).

Part B: In Vitro Cell-Based Luciferase Assay

1. Seed Akaluc-expressing cells in a white or black opaque 96-well plate at an appropriate density (typically 10,000–50,000 cells/well) 24 h before the assay. Allow cells to reach 60–80% confluency.
2. Prepare a working solution of AkaLumine in complete culture medium at the desired concentration (suggested starting range: 1–100 µM; optimize empirically for your cell line and luciferase expression level).
3. Remove existing medium from wells. Add 100 µL of AkaLumine working solution per well.
4. Incubate at 37 °C, 5% CO₂ for 10–30 minutes to allow substrate uptake and reaction equilibration.
5. Measure luminescence immediately using a luminometer or plate reader. Record relative light units (RLU). For kinetic measurements, read at multiple time points.
6. Include negative controls (non-expressing parental cells + AkaLumine) and substrate-only controls (expressing cells + vehicle) in every experiment.

Part C: In Vivo Bioluminescence Imaging in Rodents

1. Prepare the AkaLumine injection solution: dilute DMSO stock into sterile PBS to achieve the desired dose concentration. A commonly used dose range is 1–10 mg/kg body weight administered intraperitoneally (i.p.) or intravenously (i.v.); optimize for your model. Flag: exact dose should be validated per institutional IACUC protocol.
2. Filter the injection solution through a 0.22 µm syringe filter immediately before use.
3. Anesthetize the animal using isoflurane (2–3% induction, 1.5–2% maintenance in oxygen) in accordance with your institutional animal care guidelines.
4. Administer AkaLumine solution via i.p. or i.v. injection using an appropriate gauge needle.
5. Place the anesthetized animal in the imaging chamber of the NIR-capable in vivo optical imaging system. Position the animal dorsal or ventral side up depending on the anatomical region of interest.
6. Begin image acquisition 5–15 minutes post-injection (signal typically peaks within this window; optimize timing empirically).
7. Acquire images using a 650–750 nm bandpass filter (or open filter for maximum photon collection, depending on system and background). Set exposure time based on signal intensity (start with 1–5 minutes for weak signals).
8. Quantify bioluminescent signal using the imaging system software. Express results as total flux (photons/second) over the region of interest (ROI).
9. After imaging, allow the animal to recover from anesthesia in a warm environment and monitor per institutional protocol.

Troubleshooting

Data Analysis and Reporting

Quantification: Use the region-of-interest (ROI) analysis tool in your imaging software to draw ROIs over the bioluminescent foci. Report signal as total flux (photons/second, p/s) normalized to ROI area if comparing across different-sized regions. For longitudinal studies, use the same ROI dimensions and position at each time point.

Normalization: Normalize in vitro RLU values to cell number (e.g., by parallel CellTiter-Glo or crystal violet assay) or protein concentration to account for differences in cell density. For in vivo data, report body weight and injection dose for each animal.

Statistical analysis: Present data as mean ± SEM or mean ± SD with n ≥ 3 biological replicates. Use appropriate statistical tests (unpaired t-test for two groups; one-way ANOVA with post-hoc correction for multiple groups). Report exact p-values.

Controls to include in every experiment:

• Negative control: parental (non-Akaluc-expressing) cells or wild-type animals receiving AkaLumine
• Substrate control: Akaluc-expressing cells or animals receiving vehicle only
• Positive control (if available): known cell number or standard bioluminescence reference

Image display: Apply a consistent pseudocolor scale (minimum and maximum) across all images within the same experiment. State the color scale range and exposure time in figure legends.